Bacteria are responsible for the majority of surgical infections. Specific species are identified using Gram’s stain and growth characteristics on specific media. The Gram’s stain is an important evaluation that allows rapid classification of bacteria by color. This color is related to the staining characteristics of the bacterial cell wall: gram-positive bacteria stain blue and gram-negative bacteria stain red. Bacteria are classified based upon a number of additional characteristics including morphology (cocci and bacilli), the pattern of division [e.g., single organisms, groups of organisms in pairs (diplococci), clusters (staphylococci), and chains (streptococci)], and the presence and location of spores.
Gram-positive bacteria that frequently cause infections in surgical patients include aerobic skin commensals (Staphylococcus aureus and epidermidis and Streptococcus pyogenes) and enteric organisms such as Enterococcus faecalis and faecium. Aerobic skin commensals cause a large percentage of surgical site infections (SSIs), either alone or in conjunction with other pathogens; enterococci can cause nosocomial infections [urinary tract infections (UTIs) and bacteremia] in immunocompromised or chronically-ill patients, but are of relatively low virulence in healthy individuals.
There are many pathogenic gram-negative bacterial species that are capable of causing infection in surgical patients. Most gram-negative organisms of interest to the surgeon are bacilli belonging to the family Enterobacteriaceae, including Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, and Enterobacter, Citrobacter, and Acinetobacter spp. Other gram-negative bacilli of note include Pseudomonas spp., including Pseudomonas aeruginosa and fluorescens and Xanthomonas spp.
Anaerobic organisms are unable to grow or divide poorly in air, as most do not possess the enzyme catalase, which allows for metabolism of reactive oxygen species. Anaerobes are the predominant indigenous flora in many areas of the human body, with the particular species dependent on the site. For example, Propionibacterium acnes and other species are a major component of the skin microflora and cause the infectious manifestation of acne. As noted above, large numbers of anaerobes contribute to the microflora of the oropharynx and colorectum.
Infection due to Mycobacterium tuberculosis was once one of the most common causes of death in Europe, causing one in four deaths in the seventeenth and eighteenth centuries. In the nineteenth and twentieth centuries, thoracic surgical intervention was often required for severe pulmonary disease, now an increasingly uncommon occurrence in developed countries. This organism and other related organisms (M. avium-intracellulare and M. leprae) are known as acid-fast bacilli. Other acid-fast bacilli include Nocardia spp. These organisms typically are slow-growing, sometimes necessitating observation in culture for weeks to months prior to final identification, although DNA-based analysis can provide a means for preliminary, rapid detection.
