The Morphology and Fine Structure of Bacteria
Bacterial cells are between 0.3 and 5 lm in size. They have three basic forms: cocci, straight rods, and curved or spiral rods (Table 1). The nucleoid consists of a very thin, long, circular DNA molecular double strand that is not surrounded by a membrane. Among the nonessential genetic structures are the plasmids. The cytoplasmic membrane harbors numerous proteins such as permeases, cell wall synthesis enzymes, sensor proteins, secretion system proteins, and, in aerobic bacteria, respiratory chain enzymes. The membrane is surrounded by the cell wall, the most important element of which is the supporting murein skeleton (Figure 1a). The cell wall of Gram-negative bacteria features a porous outer membrane into the outer surface of which the lipopolysaccharide responsible for the pathogenesis of Gram-negative infections is integrated (Figure 1c.). The cell wall of Gram-positive bacteria does not possess such an outer membrane. Its murein layer is thicker and contains teichoic acids and wall-associated proteins that contribute to the pathogenic process in Gram-positive infections (Figure 1b.). Many bacteria have capsules made of polysaccharides that protect them from phagocytosis. Attachment pili or fimbriae facilitate adhesion to host cells. Motile bacteria possess flagella. Foreign body infections are caused by bacteria that form a biofilm on inert surfaces. Some bacteria produce spores, dormant forms that are highly resistant to chemical and physical noxae.
Figure 1. Bacterial cell structure (a), cell wall structure of Gram+ (b) and Gram- (c) bacterial.
Table 1. Morphological Characteristics of Bacteria.
|Occur in clusters, chains, pairs (diplococci), packets
Uniform thickness, rounded ends, pointed ends, club form
Comma shaped, spiral, screw shaped
Bacteria without a rigid cell wall; coccoid cells, long threads
Two forms: spherical/oval elementary bodies (300 nm);
spherical/oval reticulate bodies (1000 nm)
Short coccoid rods (0.3–1 lm)
The pathogenesis of bacterial infection includes initiation of the infectious process and the mechanisms that lead to the development of signs and symptoms of disease. The biochemical, structural, and genetic factors play important roles in bacterial pathogenesis. Characteristics of bacteria that are pathogens include transmissibility, adherence to host cells, persistence, invasion of host cells and tissues, toxigenicity, and the ability to evade or survive the host’s immune system. Resistance to antimicrobials and disinfectants can also contribute to virulence, or an organism’s capacity to cause disease. Many infections caused by bacteria that are commonly considered to be pathogens are inapparent or asymptomatic. Disease occurs if the bacteria or immunologic reactions to their presence cause sufficient harm to the person.
Bacterial Birulence Factors
Virulent bacteria cause disease through the elaboration of factors that facilitate adherence, persistence, invasion, and toxigenicity. Genes that encode virulence factors may be carried on mobile genetic elements such as plasmids or bacteriophages or are found on large pathogenicity islands on bacterial chromosomes.