Although bacteria generally lack internal membranes, they are highly sophisticated cells whose organization and behaviors have attracted the attention of many scientists. Bacteria are classified broadly by their shape—as rods, spheres (cocci), or spirals (Figure 23–3A)— as well as by their so-called Gram-staining properties, which reflect differences in the structure of the bacterial cell wall. Gram-positive bacteria have a thick layer of peptidoglycan cell wall outside their inner (plasma) membrane (Figure 23–3B), whereas Gram-negative bacteria have a thinner peptidoglycan cell wall. In both cases, the cell wall protects against lysis by osmotic swelling, and it is a target of host antibacterial proteins such as lysozyme and antibiotics such as penicillin. Gram-negative bacteria are also covered outside the cell wall by an outer membrane containing lipopolysaccharide (LPS) (Figure 23–3C). Both peptidoglycan and LPS are unique to bacteria and are recognized as pathogen-associated molecular patterns (PAMPs) by the host innate immune system, as discussed in Chapter 24. The surface of bacterial cells can also display an array of appendages, including flagella and pili, which enable bacteria to swim or adhere to desirable surfaces, respectively (Figure 23–3D). Apart from cell shape and structure, differences in ribosomal RNA and genomic DNA sequence are also used for phylogenetic classification. Because bacterial genomes are small—typically between 1,000,000 and 5,000,000 nucleotide pairs (compared to more than 3,000,000,000 for humans)—they are now simple to sequence, making this an important new classification tool.
Bacteria also exhibit extraordinary molecular, metabolic, and ecological diversity. At the molecular level, bacteria are far more diverse than eukaryotes, and they can occupy ecological niches having extremes of temperature, salt concentrations, and nutrient limitation. Some bacteria replicate in an environmental reservoir such as water or soil and only cause disease if they happen to encounter a susceptible host; these are called facultative pathogens. Others can only replicate inside the body of their host and are therefore called obligate pathogens. Bacteria also differ in the range of hosts they will infect. Shigella flexneri, for example, which causes epidemic dysentery (bloody diarrhea), will infect only humans and other primates. By contrast, the closely related bacterium Salmonella enterica, which is a common cause of food poisoning in humans, can also infect other vertebrates, including chickens and turtles. A champion generalist is the opportunistic pathogen Pseudomonas aeruginosa, which can cause disease in a wide variety of plants and animals.
Pathogenic bacteria and their closest nonpathogenic relatives often differ in a relatively small number of genes. Genes that contribute to the ability of an organism to cause disease are called virulence genes, and the proteins they encode are called virulence factors. Such virulence genes are often clustered together on the bacterial chromosome; large clusters are called pathogenicity islands. Virulence genes can also be carried on bacteriophages (bacterial viruses) or transposons (see Table 5–4), both of which integrate into the bacterial chromosome, or on extrachromosomal virulence plasmids (Figure 23–4A).