Nocardia, Streptomyces, Rhodococcus, and Similar Organisms

Published on 08/02/2015 by admin

Filed under Basic Science

Last modified 22/04/2025

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 5 (1 votes)

This article have been viewed 6478 times

Nocardia, Streptomyces, Rhodococcus, and Similar Organisms

Objectives

1. Describe the general characteristics of the aerobic actinomycetes, including their Gram stain morphology, microscopic morphology, colonial morphology, and biochemical reactions.

2. Describe the habitats of actinomycetes and the routes of transmission.

3. Describe the three types of skin infections caused by Nocardia spp. in immunocompromised individuals.

4. List the laboratory tests used to differentiate the clinically relevant aerobic actinomycetes.

5. List the laboratory tests used to differentiate the pathogenic Nocardia spp.

6. Describe the chemical structures required for an organism to be classified as acid-fast.

7. List the virulence factors associated with Nocardia asteroides.

8. Define mycetoma and actinomycetoma.

9. List the various selective media used to isolate aerobic actinomycetes and describe their usefulness in achieving optimal recovery.

Genera and Species to Be Considered

The actinomycetes are a large and diverse group of gram-positive bacilli. For the most part, cells of all actinomycetes elongate to form branching, filamentous forms. The rate and extent of filament elongation with lateral branching depends on the strain of actinomycetes, the growth medium, and the temperature of incubation. Some organisms form filaments, or hyphae, on the agar surface or into the agar, whereas others produce hyphae that extend into the air.

These organisms are aerobic, facultatively anaerobic, or obligately anaerobic; only the aerobic actinomycetes are discussed in this chapter. Aerobic actinomycetes belong to the order Actinomycetales. Actinomycetes comprise more than 40 genera, but only the clinically relevant aerobic actinomycetes genera are considered here (Table 19-1). In this chapter, only aerobic actinomycetes that exhibit branching and/or partial acid-fastness are addressed. Although both the Corynebacterium and Mycobacterium genera belong to the order Actinomycetales, Corynebacterium spp. do not usually exhibit branching filaments or partial acid-fastness, and Mycobacterium spp. do not exhibit branching and are strongly (acid-alcohol) acid-fast; for these reasons, the Corynebacteriaceae and Mycobacteriaceae are addressed in Chapters 17 and 43, respectively. Another clinically significant aerobic actinomycete is Tropheryma whipplei; because this organism has not been cultured on artificial media, it is reviewed in Chapter 44. For purposes of discussion, the remaining genera of aerobic actinomycetes are divided into the two large groups: those with cell walls that contain mycolic acid and are therefore partially acid-fast and those with cell envelopes that do not contain mycolic acid and therefore are non–acid-fast.

In general, the aerobic actinomycetes are not frequently isolated in the clinical laboratory; nevertheless, these organisms are causes of serious human disease. Not only are infections caused by these organisms difficult to recognize in the clinical laboratory, the organisms are also difficult to isolate. Further complications include difficulty classifying, identifying, and performing antibiotic susceptibilities on aerobic actinomycetes isolated from clinical specimens. At the time of this writing, the taxonomy of the aerobic actinomycetes is complex and continues to evolve. New and reliable methods that can identify cell wall amino acids and sugars and characterize mycolic acid, menaquinones, and phospholipids in conjunction with nucleic acid phylogenetic studies are proving extremely useful for resolving the taxonomy of the actinomycetes.

General Characteristics

The genera Nocardia, Rhodococcus, Gordonia, and Tsukamurella are partially acid-fast aerobic actinomycetes. Nocardia and Rhodococcus belong to the family Nocardiaceae, and Gordonia and Tsukamurella are in the Gordoniaceae and Tsukamurellaceae families, respectively. However, the variability associated with the classification of an organism as partially acid-fast depends on the particular strain and culture conditions. Therefore, this characteristic should be interpreted with caution. The genus Actinomadura includes approximately 67 species and subspecies, with significant variation. The cell walls of this group contain the sugar madurose, a characteristic shared with the genus Dermatophilus.

Partially Acid-Fast Aerobic Actinomycetes

Nocardia spp.

Organisms belonging to the genus Nocardia are gram positive (often with a beaded appearance), variably acid-fast, catalase positive, and strictly aerobic. As they grow, Nocardia spp. form branched filaments that extend along the agar surface (substrate hyphae) and into the air (aerial hyphae). As the organisms age, nocardiae fragment into pleomorphic rods or coccoid elements. Nocardiae also are characterized by the presence of mesodiaminopimelic acid (DAP) and the sugars arabinose and galactose in peptidoglycan in the cell wall.

Currently, the taxonomy in the genus Nocardia is changing rapidly. Recognition and description of new species continue and remain controversial regarding the number of validly described species; recent publications cite 22 to 30 valid species. Of significance, Cloud et al.1 reported that the most commonly identified species was Nocardia cyriacigeorgica, not N. asteroides, as determined by partial 16S rRNA DNA sequencing, followed by N. farcinica, N. nova, N. africana, and N. veterana. The species considered human pathogens or that have been implicated as human pathogens are listed in Box 19-1. N. asteroides, N. nova, N. farcinica, N. brasiliensis, N. otitidiscaviarum (formerly N. caviae), N. pseudobrasiliensis, and N. transvalensis account for most of the diseases in humans caused by Nocardia spp.

Box 19-1   Nocardia spp. Considered Human Pathogens or Have Been Implicated in Human Disease

Rhodococcus, Gordonia, Tsukamurella spp.

Organisms belonging to the Rhodococcus, Gordonia, and Tsukamurella genera are similar to Nocardia spp. in that they are gram-positive, aerobic, catalase-positive, partially acid-fast, branching, filamentous bacteria that can fragment into rods and cocci. The extent of acid-fastness depends on the amount and complexity of mycolic acids in the organism’s cell envelope and on culture conditions. The differentiation of these three genera, as well as species identification, is difficult. In particular, the genus Rhodococcus consists of a very diverse group of organisms in terms of morphology, biochemical characteristics, and ability to cause disease. As previously mentioned, the taxonomy of these organisms continues to evolve; species included in these three genera, as of this writing, are summarized in Table 19-2.

TABLE 19-2

Species Included in the Genera Rhodococcus, Gordonia, and Tsukamurella

Genus Species
Rhodococcus equi, erythropolis, rhodnii, rhodochrous (other species of unknown significance include globerulus, marinonascens, and ruber)
Gordonia aichiensis, bronchialis, polyisoprenivorans, rubripertincta, sputi, terrae (remaining species isolated from environmental sources)
Tsukamurella paurometabola, pulmonis, tyrosinosolvens, strandjordae (T. ichonensis, T. wratislaviensis isolated from nature)

Data compiled from Brown JM et al: In Murray PR, Baron EJ, Pfaller MA et al, editors: Manual of clinical microbiology, ed 10, Washington, DC, 2003, American Society for Microbiology; Goodfellow M, Chun J, Stubbs S et al: Lett Appl Microbiol 19:401, 1994; Klatte S, Rainey FA, Kroppenstedt RM: Int J Syst Bacteriol 44:769, 1994; Lasker BA, Brown JM, McNeil MM: Clin Infect Dis 15:233, 1992; Maertens J et al: Clin Microbiol Infect 4:51, 1998; Riegel P et al: J Clin Microbiol 34:2045, 1996; Yassin AF, Rainey FA, Burrghardt J et al: Int J Syst Bacteriol 47:607, 1997; Arenskötter M et al: Appl Environ Microbiol 70:3195, 2004

Non–acid-Fast Aerobic Actinomycetes: Streptomyces, Actinomadura, Dermatophilus, Nocardiopsis, and the Thermophilic Actinomycetes

The non–acid-fast aerobic actinomycetes (i.e., Streptomyces, Actinomadura, Dermatophilus, Nocardiopsis, and the thermophilic actinomycetes) are gram-positive, branching filaments that do not contain mycolic acids in their cell envelopes and are therefore non–acid-fast. This group of actinomycetes is heterogeneous and is encountered infrequently in the clinical laboratory. Only the non–acid-fast actinomycetes associated with human disease are addressed (Table 19-3).

TABLE 19-3

Non–Acid-Fast Aerobic Actinomycetes Associated with Human Disease

Genus Number of Species Species Associated with Human Disease
Streptomyces >3000

Actinomadura 27

Dermatophilus 2 Nocardiopsis 8

image

Another group of non–acid-fast actinomycetes, the thermophilic actinomycetes, are associated with infections in humans and include the medically relevant genera Thermoactinomyces, Saccharomonospora, and Saccharopolyspora.

Epidemiology and Pathogenesis

Partially Acid-Fast Aerobic Actinomycetes

Nocardia spp.

Nocardia organisms are normal inhabitants of soil and water and are primarily responsible for the decomposition of plant material. Infections caused by Nocardia spp. are found worldwide. Because they are ubiquitous, isolation of these organisms from clinical specimens does not always indicate infection. Rather, isolation may indicate colonization of the skin and upper respiratory tract or laboratory contamination, although the latter is rare. Nocardia infections can be acquired either by traumatic inoculation or inhalation. N. asteroides sensu stricto type VI is evenly distributed throughout the United States, as is N. farcinica. The prevalence of other species varies regionally; N. brasiliensis is associated with tropical climates and has a higher prevalence in the southwestern and southeastern United States.

Nocardia spp., particularly N. asteroides, are facultative intracellular pathogens capable of growth in various human cells. The mechanisms of pathogenesis are complex and not completely understood. However, the virulence of N. asteroides appears to be associated with several factors, such as stage of growth at the time of infection, resistance to intracellular killing, tropism for neuronal tissue, and ability to inhibit phagosome-lysosome fusion; other characteristics, such as production of large amounts of catalase and hemolysins, may also be associated with virulence.

Rhodococcus, Gordonia, Tsukamurella spp.

Rhodococcus, Gordonia, and Tsukamurella spp. can be isolated from several environmental sources, especially soil and farm animals, as well as from fresh water and salt water. The organisms are believed to be acquired primarily by inhalation. For the most part, these aerobic actinomycetes are infrequently isolated from clinical specimens.

To date, Rhodococcus equi has been the organism most commonly associated with human disease, particularly in immunocompromised patients, such as those infected with the human immunodeficiency virus (HIV). R. equi is a facultative intracellular organism that can persist and replicate within macrophages. Determinants of the virulence of R. equi are under investigation and may involve cell wall mycolic acids that may play a role in intracellular survival, production of interleukin-4, and granuloma formation. Although Gordonia spp. and Tsukamurella are able to cause opportunistic infections in humans, little is known about their pathogenic mechanisms.

Non–acid-Fast Aerobic Actinomycetes: Streptomyces, Actinomadura, Dermatophilus, Nocardiopsis, and the Thermophilic Actinomycetes

Aspects of the epidemiology of the non–acid-fast aerobic actinomycetes are summarized in Table 19-4. Little is known about how these agents cause infection.

TABLE 19-4

Epidemiology of the Non–Acid-Fast Aerobic Actinomycetes

Organism Habitat (Reservoir) Distribution Routes of Primary Transmission
Streptomyces somaliensis Sandy soil Africa, Saudi Arabia, Mexico, South America Penetrating wound/abrasions in the skin
S. anulatus Soil Most common isolate in United States Penetrating wound/abrasions in the skin
Actinomadura madurae Soil Tropical and subtropical countries Penetrating wound/abrasions in the skin
A. pelletieri, A. latina Unknown, possibly soil Tropical and subtropical countries Penetrating wound/abrasions in the skin
Dermatophilus congolensis Unknown; skin commensal or saprophyte in soil(?) Worldwide, but more prevalent in humid, tropical, and subtropical regions Trauma to the epidermis caused by insect bites and thorns; contact with tissues of infected animals through abrasions in the skin
Nocardiopsis dassonvillei* Unknown Unknown Unknown
Thermophilic actinomycetes Ubiquitous; water, air, soil, compost piles, dust, hay Worldwide Inhalation

image

*Only a few cases of infection identified in the literature.

Spectrum of Disease

Partially Acid-Fast Aerobic Actinomycetes

The partially acid-fast actinomycetes cause various infections in humans.

Nocardia spp.

Infections caused by Nocardia spp. can occur in immunocompetent and immunocompromised individuals. N. asteroides, N. brasiliensis, and N. otitidiscaviarum are the major causes of these infections, with N. asteroides causing greater than 80% of infections.

Nocardia spp. cause three types of skin infections in immunocompetent individuals:

Of note, N. brasiliensis is the predominant cause of these skin infections.

In immunocompromised individuals, Nocardia spp. can cause invasive pulmonary infections and disseminated infections. Patients receiving systemic immunosuppression, such as transplant recipients, individuals with impaired pulmonary immune defenses, and intravenous drug abusers, are examples of immunosuppressed patients at risk for these infections. Patients with pulmonary infections caused by Nocardia spp. can exhibit a wide range of symptoms, from an acute to a more chronic presentation. Unfortunately, no specific signs indicate pulmonary nocardiosis. Patients usually appear systemically ill, with fever, night sweats, weight loss, and a productive cough that may be bloody. Pulmonary infection can lead to complications such as pleural effusions, empyema, mediastinitis, and soft tissue infection. An acute inflammatory response follows infection, resulting in necrosis and abscess formation; granulomas are not usually formed.

Nocardia spp. can often spread hematogenously throughout the body from a primary pulmonary infection. Disseminated infection can result in lesions in the brain and skin; hematogenous dissemination involving the central nervous system is particularly common, occurring in about 30% of patients. Disseminated nocardiosis has a very poor prognosis.

Rhodococcus, Gordonia, Tsukamurella spp.

The types of infections caused by Rhodococcus, Gordonia, and Tsukamurella spp. are listed in Table 19-5. For the most part, these organisms are considered opportunistic pathogens, because most infections occur in immunocompromised individuals.

Gordonia spp. Tsukamurella spp.

image

Non–acid-Fast Aerobic Actinomycetes: Streptomyces, Actinomadura, Dermatophilus, Nocardiopsis, and the Thermophilic Actinomycetes

Infection caused by the non–acid-fast aerobic actinomycetes is usually associated with chronic, granulomatous lesions of the skin referred to as mycetomas. Mycetoma is an infection of subcutaneous tissues that results in tissue swelling and drainage of the sinus tracts. These infections are acquired by traumatic inoculation of organisms (usually in the lower limbs) and are usually caused by fungi. If mycetoma is caused by an actinomycete, the infection is called actinomycetoma.

Except for the thermophilic actinomycetes, most of these agents have rarely been associated with other types of infections (Table 19-6). These nonmycetomic infections have occurred in immunosuppressed patients, such as those infected with HIV.

TABLE 19-6

Clinical Manifestations of Infections Caused by Non–Acid-Fast Aerobic Actinomycetes

Organism Clinical Manifestations
Streptomyces spp. (S. somaliensis and other species such as S. anulatus and S. albus) Actinomycetoma
Other (rare): pericarditis, bacteremia, and brain abscess
Actinomadura spp. (A. madurae, A. pelletieri, and A. latina) Actinomycetoma
Other (rare): peritonitis, wound infection, pneumonia, and bacteremia
Dermatophilus congolensis Exudative dermatitis with scab formation (dermatophilosis)
Nocardiopsis dassonvillei Actinomycetoma and other skin infections

The thermophilic actinomycetes are responsible for hypersensitivity pneumonitis, an allergic reaction to these agents. This is an occupational disease that occurs in farmers, factory workers, and others who are repeatedly exposed to these agents. The disease has acute and chronic forms. Patients with acute hypersensitivity pneumonitis experience malaise, sweats, chills, loss of appetite, chest tightness, cough, and fever within 4 to 6 hours after exposure; typically symptoms resolve within a day. Under some circumstances involving continued exposure to the organisms, patients suffer from a chronic form of disease in which symptoms progressively worsen with subsequent development of irreversible lung fibrosis.

Laboratory Diagnosis

Specimen Collection, Transport, and Processing

Appropriate specimens should be collected aseptically from affected areas. For the most part, no special requirements are needed for specimen collection, transport, or processing of the organisms discussed in this chapter (refer to Table 5-1 for general information). When nocardiosis is clinically suspected, multiple specimens should be submitted for culture, because smears and cultures are simultaneously positive in only a third of the cases. The significance of random isolation of Nocardia spp. from the respiratory tract is questionable, because these organisms are so widely distributed in nature. Some of the actinomycetes tend to grow as a microcolony in tissues, leading to the formation of granules. Most commonly, these granules are formed in actinomycetomas, such as those caused by Nocardia, Streptomyces, Nocardiopsis, and Actinomadura spp. Therefore, material from draining sinus tracts is an excellent specimen for direct examination and culture.

Direct Detection Methods

Direct microscopic examination of Gram-stained preparations of clinical specimens is of utmost importance in the diagnosis of infections caused by the aerobic actinomycetes. Often, the demonstration of gram-positive, branching or partially branching beaded filaments provides the first clue to the presence of an aerobic actinomycete (Figure 19-1). Unfortunately, the actinomycetes do not always exhibit such characteristic morphology; many times these organisms are not seen at all or appear as gram-positive cocci, rods, or short filaments. Nevertheless, if gram-positive, branching or partially branching organisms are observed, a modified acid-fast stain should be performed (i.e., 1% sulfuric acid rather than 3% hydrochloric acid as the decolorizing agent) (see Procedure 19-1 on the Evolve site). The modified acid-fast stain is positive in only about half of these smears showing gram-positive beaded, branching filaments subsequently confirmed as Nocardia sp. Histopathologic examination of tissue specimens using various histologic stains, such as Gomori’s methenamine-silver (GMS) stain, can also detect the presence of actinomycetes.

image
Figure 19-1 A, Gram stain of sputum obtained from a patient with pulmonary nocardiosis caused by Nocardia asteroides. B, The same sputum stained with a modified acid-fast stain. The organism is indicated by the arrow.

Procedure 19-1   Partially Acid-Fast Stain for Identification of Nocardia spp.

It is important to examine any biopsy or drainage material from actinomycetomas for the presence of granules. If observed, the granules are washed in saline, emulsified in 10% potassium hydroxide or crushed between two slides, Gram stained, and examined microscopically for the presence of filaments.

Molecular Diagnostics

Amplification techniques (i.e., polymerase chain reaction [PCR]) involving the 16srRNA sequence have been used to examine the relatedness among the genera and species within the non–acid-fast aerobic actinomycetes and thermophilic actinomycetes. When the MicroSeq System was used for identification (see Chapter 8), almost 15% of isolates were identified as Nocardia spp., but no definitive species were given. PCR paired with restriction endonuclease analysis has been used to identify commonly isolated Nocardia spp. Housekeeping heat shock protein genes coupled with the 16srRNA sequence are used in this assay. DNA sequencing of several genes, including the 16srRNA, a heat shock protein gene, and a housekeeping gene are referred to as secA1. These methods currently are not available in the clinical laboratory; they are predominantly used for taxonomic, epidemiologic, and research studies.

Cultivation

Many of the aerobic actinomycetes do not have complex growth requirements; they are able to grow on routine laboratory media, such as sheep blood, chocolate, Sabouraud dextrose, and brain-heart infusion agar. However, because many of the aerobic actinomycetes grow slowly, they may be overgrown by other normal flora present in contaminated specimens. This is particularly true for the nocardiae that require a minimum of 48 to 72 hours of incubation before colonies become visible. Because of their slow growth and the possibility of being overgrown with contaminating flora, various selective media have been used to recover nocardiae. A solid medium using paraffin as the sole source of carbon has been effective for isolating Nocardia spp. and rapidly growing mycobacteria from contaminated clinical specimens. Selective media formulated for the isolation of Legionella spp. from contaminated specimens, such as buffered charcoal-yeast extract medium with polymyxin, anisomycin, and vancomycin, have been successful in the recovery of nocardiae from contaminated specimens. Martin Lewis and colistin-nalidixic acid media also have been used. Nocardia spp. grow well on Sabouraud dextrose agar and on fungal media containing cycloheximide, such as Mycosel. Because Nocardia organisms are able to withstand the decontamination procedures used to isolate mycobacteria, isolates may be identified on mycobacterial culture media.

If other aerobic actinomycetes are considered, a selective medium, such as brain-heart infusion agar with chloramphenicol and cycloheximide, is recommended in addition to routine agar to enhance isolation from contaminated specimens. Although most aerobic actinomycetes grow at 35°C, recovery is increased at 30°C. Therefore, selective and nonselective agars should be incubated at 35°C and 30°C. Plates should be incubated for 2 to 3 weeks. The typical Gram-stain morphology and colonial appearance of the aerobic actinomycetes are summarized in Table 19-7. Examples of Gram stains and cultures of different aerobic actinomycetes are shown in Figures 19-2 and 19-3.

TABLE 19-7

Typical Gram-Stain Morphology and Colonial Appearance

Organism Gram Stain* Colonial Appearance on Routine Agar
Nocardia spp. Branching, fine, delicate filaments with fragmentation Extremely variable; adherent; some isolates are beta-hemolytic on sheep blood agar; wrinkled; often dry, chalky-white appearance to orange-tan pigment; crumbly
Rhodococcus spp. Diphtheroid-like with minimal branching or coccobacillary; colonial growth appears as coccobacilli in zigzag configuration Nonhemolytic; round; often mucoid with orange to red, salmon-pink pigment developing within 4 to 7 days (pigment may vary widely)
Gordonia spp. Nonmotile, short rods Somewhat pigmented; G. sputi: smooth, mucoid and adherent to media; G. bronchialis: dry and raised
Tsukamurella spp. Mostly long rods that fragment, no spores or aerial hyphae May have rhizoid edges, dry, white to creamy to orange
Streptomyces spp. Extensive branching with chains and spores; does not fragment easily Glabrous or waxy heaped colonies; variable morphology
Actinomadura spp. Moderate, fine, intertwining branching with short chains of spores, fragmentation White-to-pink pigment, mucoid, molar tooth appearance after 2 weeks’ incubation
Dermatophilus sp. Branched filaments divided in transverse and longitudinal planes; fine, tapered filaments Round, adherent, gray-white colonies that later develop orange pigments; often beta-hemolytic
Nocardiopsis sp. Branching with internal spores Coarsely wrinkled and folded with well-developed aerial mycelium

*Aerobic actinomycetes are gram-positive organisms that are often beaded in appearance.

Data compiled from Brown JH, Mcneil MM: In Murray PR, Baron EJ, Pfaller MA et al, editors: Manual of clinical microbiology, ed 10, Washington, DC, American Society for Microbiology, 2003; McNeil MM, Brown JM: Clin Microbiol Rev 7:357, 1994.

image
Figure 19-2 Gram stains of different aerobic actinomycetes. A, Nocardia asteroides grown on Löwenstein-Jensen medium. The arrows indicate branching rods. B, Rhodococcus equi from broth. C, R. equi grown on chocolate agar. D, Streptomyces spp. grown on Sabouraud dextrose agar.
image
Figure 19-3 Aerobic actinomycetes grown on solid media. A, Nocardia asteroides grown on Löwenstein-Jensen medium. B, Rhodococcus equi grown on chocolate agar.

Clinical laboratories are rarely asked to diagnose hypersensitivity pneumonitis caused by the thermophilic actinomycetes. These organisms grow rapidly on trypticase soy agar with 1% yeast extract. The ability to grow at temperatures of 50°C or greater is a characteristic of all thermophilic actinomycetes. Differentiation of the various agents is based on microscopic and macroscopic morphologies.

Approach to Identification

If Gram-stain morphology or colonial morphology suggests a possible actinomycetes (see Table 19-7), an acid-fast stain should be performed first to rule out rapidly growing mycobacteria (see Chapter 43), followed by a modified acid-fast stain (see Procedure 19-1). If the modified acid-fast stain results are positive, the isolate is a probable partially acid-fast aerobic actinomycete (i.e., Nocardia, Rhodococcus, Tsukamurella, or Gordonia sp). If the acid-fast stain result is negative, these organisms still are not completely ruled out because of the variability of acid-fastness among isolates belonging to this group. Aerobic actinomycetes can be initially placed into major groupings by considering the following:

Procedure 19-2   Lysozyme Resistance for Differentiating Nocardia from Streptomyces spp.

Method

1. Prepare the basal broth as follows:

Dispense 500 mL of this solution into 16 × 125-mm screw-cap glass test tubes, 5 mL per tube. Autoclave the test tubes and the remaining solution for 15 minutes at 120°C. Tighten the caps and store the tubes in the refrigerator for a maximum of 2 months.

2. Prepare the lysozyme solution as follows:

3. Add 5 mL of lysozyme solution to 95 mL of basal broth; mix gently, avoiding bubbles, and aseptically dispense in 5-mL amounts to sterile, screw-cap tubes as in step 1. Store refrigerated for a maximum of 2 weeks.

4. Place several bits of the colony to be tested into a tube of the basal glycerol broth without lysozyme (control) and into a tube of broth containing lysozyme.

5. Incubate at room temperature for up to 7 days. Observe for growth in the control tube. An organism that grows well in the control tube but not in the lysozyme tube is considered susceptible to lysozyme.

Table 19-8 summarizes the key characteristics of aerobic actinomycetes.

TABLE 19-8

Preliminary Grouping of the Clinically Relevant Aerobic Actinomycetes

Characteristics Nocardia spp. Rhodococcus spp. Gordonia spp. Tsukamurella spp. Streptomyces spp. Actinomadura spp. Dermatophilus sp. Nocardiopsis spp.
Partially acid-fast + ± ± ± − − − −
Appearance on tap water agar*: branching/aerial hyphae Extensive/+ Minimal/− Minimal/− Minimal/− Extensive/+ Variable/sparse Branching Extensive/+
Lysozyme resistance + ± − + − − − −
Urea hydrolysis + ± + + ± − + +
Nitrate reduction ± ± + − ± + − +
Growth anaerobically − − − − − − − −

image

+, predominantly positive; −, predominantly negative; ±, mostly positive with some negative isolates.

*Tap water agar: Bacto agar (Difco Laboratories, Detroit, Mich.) is added to 100 mL of tap water, sterilized, and then poured into plates.Two plates are lightly inoculated using a single streak and incubated at 30°C for up to 7 days and examined daily.

Modified from Brown JM, McNeil MM: In Murray PR, Baron EJ, Pfaller MA et al, editors: Manual of clinical microbiology, ed 10, Washington, DC, 2003, American Society for Microbiology.

Accurate identification of Nocardia to the species level is important, because differences among the species have emerged in terms of virulence, antibiotic susceptibility, and epidemiology. However, identification of the pathogenic nocardiae to the species level can be problematic, because no single method can identify all Nocardia isolates, and the methods used are time-consuming, often requiring 2 weeks. Useful phenotypic tests include the use of casein, xanthine, and tyrosine hydrolysis; growth at 45°C; acid production from rhamnose; gelatin hydrolysis; opacification of Middlebrook agar; and antimicrobial susceptibility patterns. Some of these reactions with the nocardial pathogens are summarized in Table 19-9.

TABLE 19-9

Key Tests for Differentiation of the Pathogenic Nocardia spp.

Test N. asteroides sensu stricto N. farcinica* N. nova N. travalensis (N. asteroides type IV) N. transvalensis sensu stricto N. brasiliensis N. otitidiscaviarum N. pseudobrasiliensis
Hydrolysis of:                
 Casein − − − − −/+ + − +
 Xanthine − − − − −/+ − − −
 Tyrosine − − − − −/+ + −/+ +
Growth at 42°C after 3 days ± + − − − − ± −
14-day arylsulfatase − − + NT − − − −
Acid from rhamnose ± ± − − − − − −
Gelatin hydrolysis − − − − − + − −
Opacification of Middlebrook agar 4 − + − − − − −/+ −
Api 20C assimilation:                
 Galactose − − − + + + − +
 Glycerol + + − + + + + +
 Trehalose − − − + + + + +
 Adonitol − − − − + − − −
Sensitivity by Kirby Bauer disk diffusion†:                
 Gentamicin S R S/R R/S R/S S S S
 Tobramycin S/R R S/R R R S S/R S
 Amikacin S S S S/R R S S S
 Erythromycin R S/R S R R R R R

image

NT, Not tested; +, predominantly positive; −, predominantly negative; ±, mostly positive with some negative isolates; −/+, mostly negative with some positive isolates.

*Cefotaxime resistant.

†Sensitive (S) or resistant (R) as determined by Kirby Bauer disk diffusion.

Many tests are needed to confirm the identification of the other actinomycetes at the level of speciation; these are beyond the capabilities of the routine clinical microbiology laboratory, and such cases therefore should be referred to a reference laboratory.

Antimicrobial Susceptibility Testing and Therapy

A standard for susceptibility testing by broth microdilution and with cation-supplemented Mueller-Hinton broth has been approved by the Clinical and Laboratory Standards Institute (CLISI; formerly the National Committee for Clinical and Laboratory Standards), along with interpretive guidelines. Other methods, including modified disk diffusion, agar dilution, broth microdilution, E-test, and radiometric growth index have been used for antimicrobial susceptibility testing of Nocardia spp. However, although these methods demonstrate good interlaboratory and intralaboratory agreement and reproducibility, correlation of in vitro susceptibility testing results with clinical outcome has not been systematically performed at the time of this writing. Nevertheless, antimicrobial susceptibility testing should be performed on clinically significant isolates of Nocardia spp. If required, the isolate should be sent to a reference laboratory. For all other actinomycetes, no standardized methods currently are available. In some instances, susceptibility studies of Rhodococcus and Gordonia spp. can be used as a guide for directing therapy.

The primary drugs of choice against the aerobic actinomycetes are shown in Table 19-10; no effective antimicrobial therapy is available for hypersensitivity pneumonitis caused by the thermophilic actinomycetes.

Rhodococcus, Gordonia, Tsukamurella spp. Streptomyces spp. Actinomadura spp. Nocardiopsis dassonvillei Dermatophilus congolensis

image

Prevention

No vaccines are available for the prevention of infection with aerobic actinomycetes; some have been developed, but with little success. With respect to hypersensitivity pneumonitis caused by the thermophilic actinomycetes, patients must prevent the disease by avoiding exposure to these sensitizing microorganisms.

Case Study 19-1

A 60-year-old woman with a history of steroid-treated rheumatoid arthritis presented to the emergency department with increasing confusion, lethargy, and fever, which began approximately 2 weeks before admission. She had an elevated white blood cell (WBC) count. A spinal tap was performed, which showed elevated protein, low glucose, and a WBC count of 200 mm3. The cerebrospinal fluid was cultured, and two colonies of nonhemolytic, catalase-positive, gram-positive rods grew on the second quadrant of the blood agar plate after 48 hours. The chocolate plate had no growth.

Related posts:

Upper Respiratory Tract Infections and Other Infections of the Oral Cavity and Neck Infections of the Eyes, Ears, and Sinuses Erysipelothrix, Lactobacillus, and Similar Organisms Obligate Intracellular and Nonculturable Bacterial Agents Other Protozoa Haemophilus