Bacteria are prokaryotes, organisms in which the genetic material is not separated from the cytoplasm by a nuclear membrane. Most bacterial genes exist as part of a single circular chromosome, but some are present on smaller extrachromosomal circles called plasmids, which typically determine inheritance of 1 or a few characteristics. Plasmid DNA is passed between bacterial strains and species more easily than is chromosomal DNA and represents an important mechanism in the rapid proliferation of mutations such as antibiotic resistance.
The prokaryote cell wall imparts shape and rigidity to the cell and also mediates interactions with other bacteria, bacterial viruses, and the environment, including therapeutic drugs. Bacteria are broadly characterized by their microscopic shape as either cocci (round) or bacilli (elongated, or rodlike), with some being indeterminate. Bacteria are further characterized as either gram-positive or gram-negative, according to the reaction of the cell wall (blue or red, respectively) to the Gram stain. This characteristic provides critical information on the structure and biochemical composition of the cell wall that can be predictive of the bacteria’s antibiotic susceptibility (Table 10-1). The cell wall of gram-positive bacteria consists of a thick layer of peptidoglycan, the primary target of penicillin, and teichoic acid, whereas the cell wall of gram-negative bacteria is composed of a thin layer that is covered by an external lipopolysaccharide membrane (endotoxin), which excludes certain antibiotics. Some bacteria stain poorly with Gram stain, including Mycobacteria and Nocardia asteroides, visualized best with acid-fast stain.
Table 10-1 Bacterial Classification for Gram Staining
Structures external to the cell wall facilitate bacterial interactions, including flagella (motility), pili (bacterial conjugation [transfer of bacterial DNA from one bacterial cell to another]), fimbriae (bacterial adherence), and adhesins (mucosal surface adhesion). The rapid replication times of bacteria, combined with plasmid-mediated and chromosome-mediated mutations as well as biofilm formation, favor bacterial survival and make it largely inevitable that bacteria will develop resistance to antibiotics.
Staphylococci inhabit the skin, skin glands, and mucous membranes of healthy mammals. They grow in grapelike clusters in culture but may be seen singly, in pairs, or in short chains in smears from ocular specimens. Staphylococci produce an external biofilm that interferes with phagocytosis and secrete a variety of extracellular proteins—including toxins, enzymes, and enzyme activators—that facilitate both colonization and disease induction. These bacteria adapt quickly and effectively to administered antibacterial agents and may develop resistance to β-lactams, macrolides, tetracyclines, and quinolones. Ocular and nonocular infections due to methicillin-resistant Staphylococcus aureus (MRSA) are an increasing problem, leading to the common use of vancomycin, which continues to provide reliable gram-positive coverage for ocular pathogens. Resistance to vancomycin is emerging, however, requiring the development and introduction of newer drugs.
Streptococci inhabit the mucous membranes of the normal upper respiratory tract and female genital tract (Fig 10-1) and grow in pairs and chains. The historical classification of streptococci was based on serologic grouping of their cell wall carbohydrates (Lancefield groups) and their ability to hemolyze blood-containing agar media, which is useful for initial recognition of clinical isolates. These methods are used less often today, given the availability of genetic sequence data.
Figure 10-1 Gram-positive cocci (Streptococcus pneumoniae). (Gram stain, original magnification ×1000.)
(Courtesy of James Chodosh, MD.)
Streptococcus pneumoniae appear in smears as lancet-shaped diplococci and express a polysaccharide capsule that resists phagocytosis by macrophages and neutrophils. The toxin pneumolysin is liberated by autolysis and inhibits neutrophil chemotaxis, phagocytosis, lymphocyte proliferation, and antibody synthesis.
Enterococci, seen in either pairs or short chains, are a common source of antibiotic resistance such as vancomycin-resistant enterococci (VRE). Enterococcus faecalis is an important cause of endophthalmitis.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.