Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (460 page)

Bacterial pathogens may be classified by criteria such as Gram stain characteristics (gram-positive or gram-negative), shape (cocci, bacilli, coccobacilli, curved bacilli, spiral bacteria), growth atmosphere (aerobic, anaerobic, microaerophilic, CO
₂
supplemented), optimal growth temperature (25, 35, 42°C), growth rate, inhibition on selective agar (e.g., MacConkey), required enrichment (e.g., heme, cysteine), and other factors. Definitive identification and characterization may depend on biochemical, serologic, molecular, or other testing.

Mycobacteria and other acid-fast organisms are discussed in a separate section.

   
Gram-negative bacilli, nonfastidious
: The pathogens in this group grow within 24–48 hours on routine laboratory media, like sheep blood agar (SBA). Inoculation of selective and differential media, like MacConkey (MAC) agar, may facilitate isolation from contaminated specimens. Aerobic gram-negative bacteria (GNBs) may be grouped on the basis of their ability to ferment glucose. Glucose-fermenting pathogenic GNBs include the “enterics,” like
Escherichia coli
and
Salmonella
, as well as the
Vibrio
spp. Glucose nonfermenters (nGNBs) include
Pseudomonas aeruginosa
and
Acinetobacter
spp. Gram staining demonstrates avidly staining organisms. These GNBs demonstrate a variety of resistance mechanisms. Standardized susceptibility testing is required to guide treatment for most infections caused by this group of pathogens.

   
Gram-negative bacilli, fastidious
: Organisms in this group are usually capable of growth in vitro but require enriched media or special techniques for isolation.

   
Gram-negative cocci
: Organisms in this group usually grow well and rapidly on routine laboratory media but may require chocolate or other enriched media for isolation. Selective media may be used to improve isolation from specimens likely to be contaminated with endogenous flora. Empirical therapy is usually successful, but susceptibility testing is recommended for patients who fail to respond or in regions with decreased rates of susceptibility to standard treatments. Serologic testing does not play a role in routine diagnosis or management.

   
Gram-positive bacilli
: The gram-positive bacilli (GPB) usually grow within 24–48 hours on routine laboratory media, like SBA. Inoculation of selective and differential media, like Columbia colistin–nalidixic acid (CNA) or phenylethyl alcohol agar (PEA), may facilitate isolation from contaminated specimens.

   
Gram-positive cocci
: Gram-positive cocci (GPCs) cause a wide variety of infections in immunocompromised and immunocompetent hosts. Organisms grow well and rapidly on media routinely inoculated for bacterial infections. Selective media improve detection of carriage from specimens with mixed flora, as for methicillin-resistant
Staphylococcus aureus
or vancomycin-resistant enterococci (VRE). Standardized susceptibility testing may be required for management of some infections because of unpredictable susceptibility patterns. Molecular methods are playing an increasing role in diagnosis of some infections. Serologic testing does not play a role in diagnosis of acute infection.

   
Intracellular bacterial pathogens
: These organisms are unable to proliferate independently outside of host eukaryotic cells, limiting the use of routine culture for diagnosis; some agents may grow in eukaryotic cell culture, such as used for virus isolation. Infection may be confirmed by direct detection, serologic response, or molecular diagnostic methods.

   
Spiral bacteria
: The spiral bacteria form a large, metabolically diverse group of microorganisms. The organisms in this group do not grow or are difficult to grow, in vitro. In addition, special staining techniques, like silver staining, dark-field, or immunofluorescent microscopy, are needed for direct detection in specimens. Therefore, serologic techniques play a major role in specific diagnosis of these infections. Molecular diagnostic techniques are also emerging as important diagnostic tools.

   
Cell wall
–
deficient bacteria
: These pathogens lack the rigid outer cell wall that is typical of bacteria. They do not stain by Gram staining but may be visualized by special stains, like acridine orange. Agents are not isolated by routine culture techniques; serologic testing and molecular diagnostic testing are important methods when specific diagnosis is required.

ACINETOBACTER INFECTION

   Definition

Acinetobacter baumannii
is a nonfastidious, glucose nonfermenting GNB. This species is the second most frequently isolated nGNB in the clinical laboratory, playing an important role in the etiology of nosocomial infections.

   Who Should Be Suspected?