Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell

Antisepsis, Disinfection, and Sterilization

and N. meningitidis can cause meningitis in young adults. Pseudomonas, Burkholderia Common environmental contaminants in water and soil; some strains are plant pathogens. P. aeruginosa and B. cepacia are frequently implicated in hospital-acquired infections, usually associated with proliferation in moist environments and water lines. Pseudomonads can cause biofilm fouling in industrial water lines. Facultative anaerobes Rod-shaped; can grow in the presence or absence of oxygen Enterobacteriaceae Erwinia Plant saprophytes and pathogens Escherichia E. coli is the most prevalent microorganism in the lower intestinal tract and a common cause of intestinal and urinary tract infections. It is also widely used as a cloning host in molecular biology. Salmonella Leading cause of gastroenteritis, mostly food or water borne; examples are S. enterica serovarTyphi (causing typhoid fever) and S enterica serovarTyphimurium (causing gastroenteritis and enteric fever) Yersinia Zoonotic infections; Y. pestis causes plague Vibrionaceae Vibrio Gastrointestinal diseases, including cholera (V. cholerae) and food poisoning (V. parahaemolyticus) Pasteurellaceae Haemophilus Commonly found in the upper respiratory tracts of humans and some animals; H. infiuenzae is a leading cause of meningitis in children Pasteurella Can cause septicemia in animals and humans Other gram-negative bacteria Various shapes and growth requirements Bacteroides Anaerobic rods; commonly found in the intestine and as opportunistic pathogens in wounds Veillonella Anaerobic cocci; human and animal parasites Rickettsia, Chlamydia, Chlamydophila Obligate intracellular pathogens Cyanobacteria Free-living in water; photosynthetic; can be unicellular or filamentous Myxobacteria Waterborne bacteria that are motile by a gliding mechanism Leptothrix Sheathed, filamentous bacteria associated with polluted water

Bacteria can also have a variety of other proteinaceous cell surface appendages, including pili, fimbriae, and flagella (Fig. 1.6). For example, flagellar filaments are composed primarily of flagellin protein subunits and have other proteins that interact with the cell membrane and/or cell wall structure. Flagella are specifically involved in bacterial motility. Fimbriae and pili play important roles in surface, including cell surface, interactions.

A further cell wall structure that deserves separate consideration is the mycobacterial cell wall (Fig. 1.7). Mycobacteria are aerobic, slow-growing, rod-shaped bacteria (for example, Mycobacterium tuberculosis [Fig. 1.9]), which typically stain gram positive and can be further differentiated by acid-fast stain (staining with fuchsin, which resists acid and alcohol decolorization) due to their unique hydrophobic cell wall structure.

FIGURE 1.9 Cells of Mycobacterium tuberculosis. Courtesy of Clifton Barry, NIAID.

This mycobacterial cell wall structure presents a strong permeability barrier and is responsible for the higher level of resistance to antibiotics and biocides in comparison to other bacteria. The cell membrane is similar to that described in other bacteria, which can be linked to the cell wall by glycolipids. The cell wall has a three-layer structure, consisting of a peptidoglycan layer external to the cell membrane, which is covalently linked to a specific polysaccharide (known as arabinogalactan), and finally an external mycolic acid layer. The peptidoglycan is similar to that in other bacteria, but N-acetylmuramic acid is replaced with N-glycoylmuramic acid and cross-linked by three- and four-amino-acid peptides. Arabinogalactan is a polysaccharide of arabinose and galactose. The mycolic acids are attached to arabinose residues of the arabinogalactan and are some of the longest fatty acids known in nature. In mycobacteria, they typically range in carbon length from C₆₀ to C₉₀ and can make up >50% of the cell weight. In addition, the mycobacterial cell wall can contain a variety of proteins (including enzymes), short-chain fatty acids, waxes, and LPSs. Examples are the LPS lipoarabinomannan, which plays a role in host interactions during M. tuberculosis infections, and porin proteins, with a function in molecule transport similar to that seen in the outer membranes of gram-negative bacteria. In some disease-causing mycobacteria, these may also form an external capsule containing enzymes and adherence factors that play roles in mycobacterial pathogenesis. Similar basic cell wall structures have been identified in other bacteria, including actinomycetes (Nocardia) and gram-positive rods (Corynebacterium), with notably shorter-chained mycolic acids of C₄₆ to C₆₀ and C₂₂ to C₃₂, respectively, and in some cases (Amycolatopsis) no mycolic acids. Examples of bacteria with mycobacterium-like cell wall structures are given in Table 1.11.

TABLE 1.11 Cell wall structures in mycobacteria and related organisms

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General type	Key characteristics	Example(s)	Significance
Mycobacteria	Slowly to very slowly growing; acid-fast; generally gram positive; aerobic; rod shaped but also pleomorphic or filamentous	Mycobacterium
		• Slowly growing (weeks to months)
		M. tuberculosis, M. bovis	Cause tuberculosis, a respiratory tract disease, in humans and animals
		M. leprae	Causes leprosy, a skin and nerve disease