Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell
FIGURE 1.4 Life cycle of Toxoplasma gondii.
FIGURE 1.5 Simple representation of a mycoplasma cell surface structure.
Bacteria that contain cell walls can be simply classified based on their cell morphology and general reaction to a staining method known as the Gram stain. The Gram stain is used to differentiate between two types of cell wall structures: gram positive and gram negative. Microscopic examination of stained preparations allows further differentiation based on their shape (Table 1.8). However, this is an oversimplification, as bacteria vary widely in their morphologies and staining characteristics; many other methods are used for further differentiation, including assays of oxygen requirements, growth characteristics, and lipid composition; immunoassays; and molecular biological procedures.
TABLE 1.7 Examples of pathogenic mycoplasmas
| Type | Example(s) | Significance |
| Spiroplasma | S. citri | Plant pathogens, insect parasites |
| Ureaplasma | U. urealyticum | Human parasites, genital tract diseases |
| Mycoplasma | M. genitalium, M. pneumoniae | Urethritis, atypical pneumonia |
The basic structure of cell wall-containing bacteria consists of an outer cell wall and an inner cell membrane surrounding the internal cytoplasm (Fig. 1.6). The cell surface can also contain additional structures, such as pili, flagella, and capsules, depending on the bacterial species and its growth conditions.
The cell membrane is similar to that in mycoplasmas and consists of a phospholipid bilayer (without sterols) and associated proteins. Membrane proteins can be at the interface with the cytoplasm, embedded within the membrane, and/or associated with the external wall of the cell. Examples are some lipoproteins (proteins with lipid groups attached), in which the lipid component allows anchoring to the membrane. The overall structure is fluid but serves as a barrier to contain the cytoplasm and to restrict the passage of nutrients and ions into and out of the cell. Membrane proteins play vital roles in many cellular activities, including transport mechanisms, enzymatic reactions, cell signaling, energy generation, and cell wall synthesis. For this reason, damage to the cell membrane can render bacteria nonviable. The cell wall structures are less similar and can be considered as three basic types: gram-positive, gram-negative, and mycobacterial cell walls (Fig. 1.7). Mycobacteria (not to be confused with the cell wall-free mycoplasmas) are considered separately due to their unique cell wall structure. The cell wall can play an important role in the resistance of bacteria to disinfection (see chapter 8).
TABLE 1.8 General differentiation of types of bacteria based on their microscopic morphologies and reactions to Gram staining
| Bacterial structure | Shape | Example(s) |
| Cocci |
|
Gram positive: Staphylococcus, Streptococcus Gram negative: Neisseria, Veillonella |
| Bacilli (rods) |
|
Gram positive: Bacillus, Listeria Gram negative: Escherichia, Pseudomonas |