Most of the organisms listed in Table 17-1 are part of the normal human flora and colonize various parts of the human body, are found in the environment, or are associated with various animals. The two most notable pathogens are Listeria monocytogenes and Corynebacterium diphtheriae. However, these two species differ markedly in epidemiology. L. monocytogenes is widely distributed in nature and occasionally colonizes the human gastrointestinal tract. Many foods are contaminated with L. monocytogenes, including milk, raw vegetables, cheese, and meats. C. diphtheriae is only carried by humans, but in rare cases it is isolated from healthy individuals. Primary transmission for C. diphtheriae is through respiratory secretions or exudates from skin lesions.

TABLE 17-1

Epidemiology

Organism	Habitat (Reservoir)	Mode of Transmission
Listeria monocytogenes	Colonizer: Animals, soil, and vegetable matter; widespread in these environments Human gastrointestinal tract	Direct contact: Ingestion of contaminated food, such as meat and dairy products Endogenous strain: Colonized mothers may pass organism to fetus. Portal of entry is probably from gastrointestinal tract to blood and in some instances from blood to meninges.
Corynebacterium diphtheriae	Colonizer: Human nasopharynx but only in carrier state; not considered part of normal flora Isolation from healthy humans is not common.	Direct contact: Person to person by exposure to contaminated respiratory droplets Contact with exudate from cutaneous lesions Exposure to contaminated objects
Corynebacterium jeikeium	Colonizer: Skin flora of hospitalized patients, most commonly in the inguinal, axillary, and rectal sites	Uncertain Direct contact: May be person to person Endogenous strain: Selection during antimicrobial therapy Introduction during placement or improper care of intravenous catheters
Corynebacterium ulcerans	Normal flora: Humans and cattle	Uncertain Zoonoses: Close animal contact, especially during summer
Corynebacterium pseudotuberculosis	Normal flora: Animals such as sheep, goats, and horses	Uncertain Zoonoses: Close animal contact, but infections in humans are rare
Corynebacterium pseudodiphtheriticum	Normal flora: Human pharyngeal and occasionally skin flora	Uncertain Endogenous strain: Access to normally sterile site
Corynebacterium minutissimum	Normal flora: Human skin	Uncertain Endogenous strain: Access to normally sterile site
Corynebacterium urealyticum	Normal flora: Human skin	Uncertain Endogenous strain: Access to normally sterile site
Leifsonia aquatica (formerly Corynebacterium aquaticum)	Environment: Fresh water	Uncertain
Corynebacterium xerosis	Normal flora: Human conjunctiva Skin Nasopharynx	Uncertain Endogenous strain: Access to normally sterile site
Corynebacterium striatum	Normal flora: Skin	Uncertain Endogenous strain: Access to normally sterile site
Corynebacterium amycolatum	Normal flora: Human conjunctiva Skin Nasopharynx	Uncertain Endogenous strain: Access to normally sterile site
Corynebacterium auris	Uncertain: Probably part of normal human flora	Uncertain Rarely implicated in human infections
Kurthia spp.	Environment	Uncertain Rarely implicated in human infections
Brevibacterium spp.	Normal flora: Human Various foods	Uncertain Rarely implicated in human infections
Dermabacter hominis	Normal flora: Human skin	Uncertain Rarely implicated in human infections
Turicella otitidis	Uncertain: Probably part of normal human flora	Uncertain Rarely implicated in human infections
Arthrobacter spp., Microbacterium spp., Cellulomonas spp., and Exiguobacterium sp.	Uncertain Probably environmental	Uncertain Rarely implicated in human infections

In contrast to these two organisms, C. jeikeium is commonly encountered in clinical specimens, mostly because it tends to proliferate as skin flora of hospitalized individuals. However, C. jeikeium is not considered to be highly virulent. The penetration of the patient’s skin by intravascular devices is usually required for this organism to cause infection.

Pathogenesis and Spectrum of Disease

L. monocytogenes, by virtue of its ability to survive within phagocytes, and C. diphtheriae, by production of an extremely potent cytotoxic exotoxin, are the most virulent species listed in Table 17-2. Not all strains of C. diphtheriae are toxin-producing strains. The toxin gene is present in strains that have acquired the gene by viral transduction. The result is the incorporation of the toxin gene into the organisms’ genome. C. diphtheriae occurs in four biotypes: gravis, intermedius, belfanti, and mitis; C. gravis causes the most severe form of disease. The biotypes can be differentiated based on colonial morphology, biochemical reactions, and hemolytic patterns on blood agar.

TABLE 17-2

Pathogenesis and Spectrum of Diseases

Organism	Virulence Factors	Spectrum of Diseases and Infections
Listeria monocytogenes	Listeriolysin O: A hemolytic and cytotoxic toxin that allows for survival within phagocytes Internalin: Cell surface protein that induces phagocytosis Act A: Induces actin polymerization on the surface of host cells, producing cellular extensions and facilitating cell-to-cell spread. Siderophores: Organisms capable of scavenging iron from human transferrin and of enhanced growth of organism.*	Systemic: Bacteremia, without any other known site of infection CNS infections: Meningitis, encephalitis, bran abscess, spinal cord infections Neonatal: Early onset: Granulomatosis infantisepticum—in utero infection disseminated systemically that causes stillbirth Late onset: Bacterial meningitis Immunosuppressed patients
Corynebacterium diphtheriae	Diphtheria toxin: A potent exotoxin that destroys host cells by inhibiting protein synthesis.	Respiratory diphtheria is a pharyngitis characterized by the development of an exudative membrane that covers the tonsils, uvula, palate, and pharyngeal wall; if untreated, life-threatening cardiac toxicity, neurologic toxicity, and other complications occur. Respiratory obstruction develops and release of toxin into the blood can damage various organs, including the heart.
	Nontoxigenic strains: Uncertain	Cutaneous diphtheria is characterized by nonhealing ulcers and membrane formation. Immunocompromised patients, drug addicts, and alcoholics. Invasive endocarditis, mycotic aneurysms, osteomyelitis, and septic arthritis*
Corynebacterium jeikeium	Unknown: Multiple antibiotic resistance allows survival in hospital setting	Systemic: Septicemia Skin infections: Wounds, rashes and nodules Immunocompromised: Malignancies, neutropenia, AIDS patients. Associated with indwelling devices such as catheters, prosthetic valves, and CSF shunts*
Corynebacterium ulcerans	Unknown	Zoonoses: Bovine mastitis Has been associated with diphtheria-like sore throat, indistinguishable from C. diphtheriae Skin infections Pneumonia
Corynebacterium pseudotuberculosis	Unknown	Zoonoses: Suppurative granulomatous lymphadenitis
Corynebacterium pseudodiphtheriticum	Unknown Some stains have been identified that are resistant to macrolides*	Systemic: Septicemia Endocarditis Pneumonia and lung abscesses; primarily in immunocompromised
Corynebacterium minutissimum	Unknown Probably of low virulence	Superficial, pruritic skin infections known as erythrasma Immunocompromised: Septicemia Endocarditis Abscess formation
Corynebacterium urealyticum	Unknown Multiple antibiotic resistance allows survival in hospital setting.	Immunocompromised and elderly: Urinary tract infections Wound infections Rarely: endocarditis, septicemia, osteomyelitis, and tissue infections
Leifsonia aquatica ( formerly Corynebacterium aquaticum)	Unknown	Immunocompromised: Bacteremia Septicemia
Corynebacterium xerosis	Unknown	Immunocompromised: Endocarditis Septicemia
Corynebacterium striatum	Unknown	Immunocompromised: Bacteremia Pneumonia and lung abscesses Osteomyelitis Meningitis
Corynebacterium amycolatum	Unknown Multiple antibiotic resistance patterns	Immunocompromised: Endocarditis Septicemia Pneumonia Neonatal sepsis
Corynebacterium auris	Unknown Multiple antibiotic resistance patterns	Uncertain disease association but has been linked to otitis media
Kurthia spp., Brevibacterium and Dermabacter sp.	Unknown	Immunocompromised: Rarely causes infections in humans Bacteremia in association with indwelling catheters or penetrating injuries
Turicella otitidis	Unknown	Uncertain disease association but has been linked to otitis media
Arthrobacter spp., Microbacterium spp., Aureobacterium spp., Cellulomonas spp., and Exiguobacterium sp.	Unknown	Uncertain disease association

AIDS, Acquired immunodeficiency syndrome; CSF, cerebrospinal fluid; CNS, central nervous system.

L. monocytogenes is ingested through contaminated food. Once the organism has been phagocytized by white blood cells, it produces listeriolysin O, the major virulence factor. Listeriolysin O in combination with phospholipases enables the organism to escape from the white blood cells and spread to the bloodstream, eventually reaching the central nervous system and the placenta.

Most of the remaining organisms in Table 17-2 are opportunistic, and infections are associated with immunocompromised patients. For this reason, whenever Corynebacterium spp. or the other genera of gram-positive rods are encountered, careful consideration must be given to their role as infectious agents or contaminants. Corynebacterium urealyticum is an up-and-coming cause of cystitis in hospitalized patients, in those who have undergone urologic manipulation, and in the elderly.

No special considerations are required for processing of most of the organisms discussed in this chapter. (Refer to Table 5-1 for general information on specimen processing.) One exception is the isolation of L. monocytogenes from placental and other tissue. Because isolating Listeria organisms from these sources may be difficult, cold enrichment may be used to enhance the recovery of the organism. The specimen is inoculated into a nutrient broth and incubated at 4°C for several weeks to months. The broth is subcultured at frequent intervals to enhance recovery.

Direct Detection Methods

Gram stain of clinical specimens is the only procedure used for the direct detection of these organisms. Most of the genera in this chapter (except Listeria, Rothia, and Oerskovia spp.) are classified as coryneform bacteria; that is, they are gram-positive, short or slightly curved rods with rounded ends; some have rudimentary branching. Cells are arranged singly, in “palisades” of parallel cells, or in pairs of cells connected after cell division to form V or L shapes. Groups of these morphologies seen together resemble and are often referred to as Chinese letters (Figure 17-1). The Gram stain morphologies of clinically relevant species are described in Table 17-3. L. monocytogenes is a short, gram-positive rod that may occur singly or in short chains, resembling streptococci.

TABLE 17-3

Gram Stain Morphology, Colonial Appearance, and Other Distinguishing Characteristics

Organism	Gram Stain	Appearance on 5% Sheep Blood Agar
Arthrobacter spp.	Typical coryneform gram-positive rods after 24 hr, with “jointed ends” giving L and V forms, and coccoid cells after 72 hr (i.e., rod-coccus cycle*)	Large colony; resembles Brevibacterium spp.
Brevibacterium spp.	Gram-positive rods; produce typical coryneform arrangements in young cultures (<24 hr) and coccoid-to-coccobacillary forms that decolorize easily in older cultures (i.e., rod-coccus cycle*)	Medium to large; gray to white, convex, opaque, smooth, shiny; nonhemolytic; cheeselike odor
Cellulomonas spp.	Irregular, short, thin, branching gram-positive rods	Small to medium; two colony types, one starts out white and turns yellow within 3 days and the other starts out yellow
CDC coryneform group F-1	Typical coryneform gram-positive rods	Small, gray to white
CDC coryneform group G^†	Typical coryneform gram-positive rods	Small, gray to white; nonhemolytic
Corynebacterium accolens	Resembles C. jeikeium	Resembles C. jeikeium
C. afermentans subsp. afermentans	Typical coryneform gram-positive rods	Medium; white; nonhemolytic; nonadherent
C. afermentans subsp. lipophilum	Typical coryneform gram-positive rods	Small; gray, glassy
C. amycolatum	Pleomorphic gram-positive rods with single cells, V forms, or Chinese letters	Small; white to gray, dry
C. argentoratense	Typical coryneform gram-positive rods	Medium; cream-colored; nonhemolytic
C. aurimucosum	Typical coryneform gram-positive rods	Slightly yellowish sticky colonies; some strains black-pigmented
C. auris	Typical coryneform gram-positive rods	Small to medium; dry, slightly adherent, become yellowish with time; nonhemolytic
C. coyleae	Typical coryneform gram-positive rods	Small, whitish and slightly glistening with entire edges; either creamy or sticky
C. diphtheriae group^‡	Irregularly staining, pleomorphic gram-positive rods	Various biotypes of C. diphtheriae produce colonies ranging from small, gray, and translucent (biotype intermedius) to medium, white, and opaque (biotypes mitis, belfanti, and gravis); C. diphtheriae biotype mitis may be beta-hemolytic; C. ulcerans and C. pseudotuberculosis resemble C. diphtheriae
C. falsenii	Typical coryneform gram-positive rods	Small; whitish, circular with entire edges, convex, glistening, creamy; yellow pigment after 72 hr
C. freneyi	Typical coryneform gram-positive rods	Whitish; dry; rough
C. glucuronolyticum	Typical coryneform gram-positive rods	Small; white to yellow, convex; nonhemolytic
C. jeikeium	Pleomorphic; occasionally, club-shaped gram-positive rods arranged in V forms or palisades	Small; gray to white, entire, convex; nonhemolytic
C. imitans	Typical coryneform gram-positive rods	Small, white to gray, glistening, circular, convex; creamy; entire edges
C. macginleyi	Typical coryneform gram-positive rods	Tiny colonies after 48 hr; nonhemolytic
C. matruchotii	Gram-positive rods with whip-handle shape and branching filaments	Small; opaque, adherent
C. minutissimum	Typical coryneform gram-positive rods with single cells, V forms, palisading and Chinese letters	Small; convex, circular, shiny, and moist
C. mucifaciens	Typical coryneform gram-positive rods	Small, slightly yellow and mucoid; circular, convex, glistening
C. propinquum	Typical coryneform gram-positive rods	Small to medium with matted surface; nonhemolytic
C. pseudodiphtheriticum	Typical coryneform gram-positive rods	Small to medium; slightly dry
C. pseudotuberculosis	Typical coryneform gram-positive rods	Small, yellowish white, opaque, convex; matted surface
C. riegelii	Typical coryneform gram-positive rods	Small, whitish, glistening, convex with entire edges; either creamy or sticky
C. simulans	Typical coryneform gram-positive rods	Grayish white; glistening; creamy
C. singulare	Typical coryneform gram-positive rods	Circular; slightly convex with entire margins; creamy
C. striatum	Regular medium to large gram-positive rods; can show banding	Small to medium; white, moist and smooth (resembles colonies of coagulase-negative staphylococci)
C. sundsvallense	Gram-positive rods, some with terminal bulges or knobs; some branching	Buff to slight yellow, sticky, adherent to agar
C. thomssenii	Typical coryneform gram-positive rods	Tiny after 24 hr; whitish, circular, mucoid and sticky
C. ulcerans	Typical coryneform gram-positive rods	Small, dry, waxy, gray to white
C. urealyticum	Gram-positive coccobacilli arranged in V forms and palisades	Pinpoint (after 48 hr); white, smooth, convex; nonhemolytic
C. xerosis	Regular medium to large gram-positive rods can show banding;	Small to medium; dry, yellowish, granular
Dermabacter hominis	Coccoid to short gram-positive rods	Small; gray to white, convex; distinctive pungent odor
Exiguobacterium acetylicum	Irregular, short, gram-positive rods arranged singly, in pairs, or short chains; (i.e., rod-coccus cycle*)	Golden yellow
Kurthia spp.	Regular gram-positive rods with parallel sides; coccoid cells in cultures >3 days old	Large, creamy or tan-yellow; nonhemolytic
Leifsonia aquatica	Irregular, slender, short gram-positive rods	Yellow
Listeria monocytogenes	Regular, short, gram-positive rods or coccobacilli occurring in pairs (resembles streptococci)	Small; white, smooth, translucent, moist; beta-hemolytic
Microbacterium spp.	Irregular, short, thin, gram-positive rods	Small to medium; yellow
Oerskovia spp.	Extensive branching; hyphae break up into coccoid to rod-shaped elements	Yellow-pigmented; convex; creamy colony grows into the agar; dense centers
Rothia spp.	Extremely pleomorphic; predominately coccoid and bacillary (broth, Figure 17-2, A) to branched filaments (solid media, Figure 17-2, B)	Small, smooth to rough colonies; dry; whitish; raised
Turicella otitidis	Irregular, long, gram-positive rods	Small to medium; white to cream, circular, convex