Bacterial Structure

Bacteria are prokaryotic unicellular (single-celled) independent organisms that can live as an individual cell or attached to others, forming colonies or groupings, such as biofilms. Each of the cells in such a colony is capable of carrying out all essential activities for survival, such as regular cellular metabolism and reproduction.

Shapes

Bacterial cell morphology displays a variety of different shapes and sizes (Figure 6.1), and the cells are generally much smaller than eukaryotic cells. According to their morphological characteristics most bacteria are classified into different basic shapes: coccus, bacillus, spirochetes (spiral or helical), and pleomorphic (see Chapter 4, Microbiological Laboratory Techniques).

• Cocci (singular, coccus) are bacteria whose overall shape is spherical or nearly spherical (Figure 6.2). Several cocci are human pathogens causing, for example, urinary tract infections, food poisoning, toxic shock syndrome, gonorrhea, some forms of meningitis, throat infections, pneumonias, and sinusitis.

FIGURE 6.2 *Staphylococcus aureus.*
This gram-positive coccus, appearing as grapelike clusters, is one of the most common causes of staph infections.

• Bacilli (singular, bacillus) are rod-shaped bacteria, some of which are endospore forming (Figure 6.3). Diseases caused by bacilli include anthrax, botulism, and tetanus, and gastrointestinal infections caused by bacilli such as Escherichia coli and Salmonella.

FIGURE 6.3 *Escherichia coli.*
This micrograph shows a Gram stain of *E. coli,* a typical gram-negative bacillus.

• Pleomorphic bacteria are bacterial species that are morphologically indistinct, depending on environmental conditions. This group includes Coccobacillus (coccobacilli), which are bacilli that are elongated as well as spherical in shape (Figure 6.4).

FIGURE 6.4 Coccobacilli.
This micrograph illustrates a Gram stain of an organism from the genus *Corynebacterium*. The cells are variable in shape, from spheres to short rods, and therefore are considered pleomorphic (cells with a variable or indistinct shape).

• Spirals occur as vibrios, spirilla, or spirochetes.

• Vibrios are curved or comma-shaped rods (Figure 6.5) and several species are human pathogens associated with gastroenteritis, cholera, food poisoning, and septicemia. Vibrio fischeri and V. harveyi are symbiotes of marine organisms such as jellyfish and produce light via bioluminescence.

FIGURE 6.5 Vibrio.
The micrograph shows a Gram stain of *Vibrio natriegens,* demonstrating the vibrioid shape, a simple curved rod. Note that not all cells appear curved, due to the viewing of the cells on the side with the convex/concave faces of the cell pointing directly at or away from the viewer.

• A spirillum (plural, spirilla) is a thick, rigid, spiral organism (Figure 6.6) that can cause rat bite fever, an uncommon but worldwide condition caused by rodent bites. These bacteria are present in the oropharyngeal flora of approximately 50% of healthy wild and laboratory rats, as well as in other rodents.

FIGURE 6.6 Spirillum.
This micrograph is taken from a Gram stain of the genus *Campylobacter,* showing the corkscrew or “snakelike” shape referred to as *spirilla.* Although a spirilla and a spirochete may appear very similar in overall shape, the spirilla often have flagella as a means of locomotion, whereas the spirochetes have a structure called an *axial filament* that facilitates movement and lack flagella.

• Spirochetes are thin, flexible spirals (Figure 6.7) and can cause leptospirosis, Lyme disease, and syphilis.

FIGURE 6.7 Spirochete.
This micrograph shows *Treponema pallidum*, the causative agent of syphilis. From Murray PR, Rosenthal KS, Pfaller MA: *Medical Microbiology,* ed 5, St. Louis, Mosby, 2005.

HEALTHCARE APPLICATION
Examples of Pathogens and Opportunistic Pathogens by Shape

Organism	Shape	Reservoir	Disease(s)
Staphylococcus aureus	Coccus	Common on skin, nose, gastrointestinal and urogenital tracts of humans	Toxin mediated: Food poisoning, scaled skin syndrome, toxic shock syndrome, folliculitis, carbuncles, impetigo, wound infections, bacteremia, and more
Staphylococcus epidermidis	Coccus	Common on human skin	Bacteremia, endocarditis, urinary tract infections, opportunistic infections of catheters, shunts, prosthetic devises, and more
Enterococcus faecalis	Coccus	Common in human gastrointestinal tract	Bacteremia, endocarditis, urinary tract infections, wound infections
Bacillus anthracis	Bacillus	Soil organism	Anthrax
Bacillus cereus	Bacillus	Soil organism	Toxin mediated: Gastroenteritis (emetic, diarrheal), ocular infections, opportunistic infections
Bacillus thuringiensis	Bacillus	None (soil, gut of caterpillars, butterflies, moths)	Gastroenteritis, opportunistic infections
Haemophilus influenzae	Pleomorphic	Mucous membranes of humans	Meningitis, epiglottitis, pneumonia, bacteremia, opportunistic infections
Chlamydia trachomatis	Pleomorphic	Obligate intracellular human pathogen	Sexually transmitted: Prostatitis, epididymitis, cervicitis, pelvic inflammatory disease, urethritis, and more
Vibrio cholerae	Spiral (vibrio)	Estuarine and marine environments	Toxin mediated: Acute diarrheal illness
Spirillum minus	Spiral (spirillum)	Rodents	Rat bite fever
Borrelia burgdorferi	Spiral (spirochete)	Vector-borne, transmitted by ticks	Lyme disease

Arrangements

In addition to their shape, bacteria can also be categorized according to their arrangement or their style of grouping after cell division.

• Aggregations of cocci occur after cell division and can be classified as the following (Figure 6.8):

FIGURE 6.8 Arrangements of cocci.
Cocci occur in five basic arrangements: diplo, in pairs; staph(ylo), in grapelike irregular clusters; strep(to), in chains; tetrads, in squares of four; and sarcinae, three-dimensional cubes. A, *Staphylococcus;* B, *Streptococcus;* C, *Micrococcus* in tetrads.

• Diplo– is a prefix typically describing cells that occur in pairs of two, joined in one plane only. Examples of diplococci are Streptococcus pneumoniae, Neisseria gonorrhoeae, and Neisseria meningitidis.

• Strepto– is a prefix used to indicate an arrangement of cells in beadlike chains, because cell division occurs along a single axis, in contrast to staphylococci, which divide along multiple axes. Individual species of Streptococcus are identified primarily on the basis of their hemolytic properties on blood agar. The species Streptococcus pyogenes is responsible for strep throat, many cases of meningitis, bacterial pneumonia, endocarditis, erysipelas (acute skin infection), and necrotizing fasciitis (“flesh-eating” infections).

• Tetrads are produced by division within two planes, with the cocci arranged in squares of four in irregular clusters. Micrococcus luteus, which can be found in many areas including the human skin, water, dust, and soil, shows this growth pattern. It is considered to be a harmless bacterium but some cases of Micrococcus infections have been reported in people with a compromised immune system (e.g., HIV patients).

• Sarcinae are cocci arranged in cubes of eight as a result of division in three planes. This coccal grouping occurs when any cocci fail to separate after they divide and the resultant daughter cells remain attached.

• Staphylo– is a prefix indicating arrangements in grapelike clusters formed by cell division in random planes. Most members of the genus Staphylococcus are harmless and are part of the normal flora of the skin and mucous membranes in humans. A small percentage of staphylococci are also part of the soil microbial flora. The organism can cause an array of diseases in humans and other animals by invasion and also by toxin production. The toxins of Staphylococcus are a common cause of food poisoning because the bacteria can grow in improperly stored food. Although the organism is killed by the cooking process, the enterotoxin is heat resistant and can survive boiling for several minutes.

• Bacilli divide in one plane, producing diplobacilli and streptobacilli (Figure 6.9).

FIGURE 6.9 Arrangements of bacilli.
Bacilli can be found in three basic arrangements: A, diplo, in pairs; B, strep(to), in chains; or palisades, lying next to each other along their long sides.

• Diplobacilli are paired rods that remain in pairs after division.

• Streptobacilli fail to separate after they divide and remain in chains of cells.

MEDICAL HIGHLIGHTS

Fusobacterium: Dental Freeloaders

Bacteria of the genus Fusobacterium have a distinct shape—that of a rod with pointed ends or a spindle shape. This is, however, not the only interesting feature about these organisms. Fusobacterium nucleatum is a human pathogen that is typically found in the oral cavity, part of the oral flora that is responsible for plaque formation and tooth decay. One of the unique features about this spindle-shaped bacterium is its ability to adhere to other plaque-forming gram-negative bacteria that have already established a biofilm on the surface of the teeth or mucous membranes. Once added to the existing biofilm, F. nucleatum shares the same environmental advantages that the original builders of the biofilm enjoy and their acidic metabolic by-products contribute to tooth decay and periodontal disease. Although linked primarily with the oral cavity, this pathogen has also been associated with infections in the digestive tract, lungs, and liver.

Bacterial Growth

Growth is an increase in the quantity of cellular material and depends on the ability of the cell to form new protoplasm from available nutrients. Bacterial growth requires a source of energy for protein synthesis and to maintain bacterial metabolism. Bacteria must obtain or synthesize nucleic acids, carbohydrates, and lipids that are used as building blocks of cells. The minimal requirements for growth are a source of carbon and nitrogen, an energy source, water, and a variety of ions and minerals. Oxygen is necessary for some bacteria but can be lethal for many others (see Atmospheric Conditions, later in this chapter).

Basis of Bacterial Growth: Binary Fission

Bacterial replication is a coordinated process accomplished primarily by binary fission. Binary fission results in two identical daughter cells (Figure 6.10). In order for growth to occur sufficient metabolites are necessary to support the synthesis of bacterial components. A cascade of regulatory events must occur to initiate replication. Once replication is started DNA synthesis must run to completion. Chromosome replication starts at the plasma membrane and each daughter chromosome is anchored to a different portion of the membrane. Membrane, peptidoglycan synthesis, and cell division are linked together and as the bacterial membrane grows, the daughter chromosomes are pulled apart. At the end of chromosome replication septum formation between the daughter cells starts, indicating cell division (see Chapter 3, Cell Structure and Function). New initiation may begin even before chromosome duplication and cell division are complete.

FIGURE 6.10 Binary fission.
Bacterial replication is a coordinated process accomplished primarily by binary fission. The circular DNA molecule is replicated, the bacterial cell elongates, and the cell splits into two identical cells, each containing an exact copy of the original DNA.

At the point at which metabolites are depleted or toxic by-products build up, chemical alarmones are produced, which stops synthesis. Degradative processes and DNA synthesis continue until all initiated chromosomes are completed. Ribosomes are broken down during the degradative process to provide deoxyribonucleotide precursors. Peptidoglycan and proteins are degraded for metabolites and the cell may shrink in response. Complete cell division may not occur and the cell may ultimately die. Under degradative processes sporulation may occur in species capable of this process.

Population Growth Curve

To produce a bacterial culture in a laboratory environment, the organism must be added to a medium (see Types of Culture Media in Chapter 4, Microbiological Laboratory Techniques). Under favorable conditions a growing bacterial culture doubles at regular intervals. Under ideal conditions, bacterial growth can be described by four different phases: the lag phase, the exponential or logarithmic growth phase, the stationary phase, and the death phase (Figure 6.11).