Phenotypic criteria are based on observable physical or metabolic characteristics of bacteria, that is, identification is through analysis of gene products rather than through the genes themselves. The phenotypic approach is the classic approach to bacterial identification, and most identification strategies are still based on bacterial phenotype. Delineation of some characteristics may require subcellular analysis involving sophisticated instrumentation (e.g., high-performance liquid chromatography [HPLC] to analyze cell wall components. For more information about these techniques, see Chapter 8). Other characterizations are based on the antigenic makeup of the organisms and involve techniques based on antigen-antibody interactions (for more information regarding immunologic diagnosis of infectious diseases, see Chapter 10). However, most of the phenotypic
characterizations used in diagnostic bacteriology are based on tests that establish a bacterial-isolate's morphology and metabolic capabilities. The most commonly used phenotypic criteria include:
Microscopic Morphology and Staining Characteristics
Microscopic evaluation of bacterial cellular morphology, as facilitated by the Gram stain or other enhancing methods discussed in Chapter 6, provides the most basic and important information on which final identification strategies are based. For this reason, a Gram stain of bacterial growth from isolated colonies on various media is usually the first step in any identification scheme. Based on these findings, most clinically relevant bacteria can be divided into four distinct groups: gram-positive cocci, gram-negative cocci, gram-positive bacilli, and gram-negative bacilli (Figure 7-13). Some bacterial species are morphologically indistinct and are described as "gram-negative coccobacilli," "gram-variable bacilli," or pleomorphic (i.e., exhibiting various shapes). Still other morphologies include curved and/or rods and spirals.
Even without staining, examination of a wet preparation of bacterial colonies under oil immersion (lOOOx magnification) can provide clues as to possible identity. For example, a wet preparation prepared from a translucent, alpha-hemolytic colony on blood agar may reveal cocci in chains, a strong indication that the bacteria are probably streptococci. Also, the
Gram stain morphology
Was this article helpful?