Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell
soft tissue infections (e.g., cellulitis).
Some unique gram-negative-staining, obligate intracellular bacteria that were previously thought to be viral in nature have been identified, including the chlamydias and rickettsias. Rickettsias are small bacteria with a simple cell wall structure, similar to gram-negative bacteria, and are pleomorphic in shape (ranging from rods to cocci). Most are transferred to humans by arthropods (ticks and lice). Typical diseases caused by rickettsias include typhus (Rickettsia prowazekii and Rickettsia typhi) and Q fever (Coxiella burnetii). The chlamydias are also small obligate parasites. They are therefore difficult to isolate in vitro, requiring cell culturing, and typically stain as gram-negative coccoid bacteria. They are a serious cause of urogenital infections (Chlamydia trachomatis) and pneumonia (Chlamydophila pneumoniae and Chlamydophila psittaci). Chlamydia cell wall structure is unique; similar to the gram-negative cell wall, it contains an inner and outer membrane and LPS, but it does not appear to have a peptidoglycan layer. As obligate parasites, the cells are very sensitive to heat, drying, and biocides.
Figure 1.6 also shows that other structures can be present on the surfaces of bacteria. Of particular interest in the consideration of biocidal processes are external barriers that can protect the cell from its environment. Many bacteria produce an external layer of highmolecular-weight polysaccharides, as well as associated lipids and proteins, which is referred to as a glycocalyx. This can be a simple, loosely associated slime layer or a more rigid, thicker, and firmly attached capsule structure. Capsules can range in structure and size, typically from one-half to five times the cell diameter in thickness. Glycocalyx production plays an important role in the development of bacterial biofilms, which are a further intrinsic resistance mechanism (see section 8.3.8). Glycocalyx structures are found in both gram-positive and gram-negative bacteria. Examples are Streptococcus mutans (in dental plaque), Streptococcus pneumoniae (in nasopharyngeal colonization), and E. coli (enteropathogenic strains that attach to epithelial cells in the intestine). In addition to direct cell protection, they can also play roles in pathogenesis, in bacterial attachment to surfaces, and in preventing drying of the cell. Other bacteria (including archaea) produce an S-layer, similar to polysaccharide capsules, which is composed of protein and glycoproteins to form an external crystalline structure; an example is the external surface of Bacillus anthracis, which produces an S-layer consisting of two protein types that is itself covered by a unique protein (poly-D-glutamic acid) capsule layer.
TABLE 1.10 Examples of gram-negative bacteria
| General type | Key characteristics | Example(s) | Significance |
| Spirochetes | Thin; helical or spiral shaped | Borrelia | Cause what are often described as tick-borne diseases in animals, humans, and birds (e.g., B. burgdorferi, implicated in Lyme disease) |
| Treponema | Cause human and animal diseases; T. pallidum causes syphilis, a persistent sexually transmitted disease | ||
| Helical, vibroid | Usually mobile; vibroid shaped | Campylobacter Helicobacter | C. jejuni causes gastroenteritis H. pylori causes peptic ulcers due to gastritis |
| Aerobic or microaerophilic rods and cocci | Diverse group of rods or cocci that use oxygen for growth | Acetobacter Bordetella Legionella | Cause food spoilageB. pertussis causes whooping cough, a respiratory disease Associated with water or moist environments; L. pneumophila causes a form of pneumonia known as Legionnaires’ disease |
| Neisseria |
Most strains are nonpathogenic and found on mucous membranes. N. gonorrhoeae causes the sexually transmitted disease |