Overview of infectious diseases in trauma patients

Microbial inoculum

One of the most commonly studied areas of infection is the number of microbes that contaminate an anatomic site and the critical threshold that is required for infection to result. All injured sites, all surgical incisions, and all invasive medical devices will have recoverable bacteria when cultured. Our environment is not sterile, and man himself has more microbial colonists than eukaryotic cells. The presence of bacteria does not uniformly determine infection, but the numbers of bacteria per gram of tissue or unit of surface area of tissue are predictive.

Classic studies of experimental and clinical wounds have created the magical 10 ⁵ organisms per gram of tissue as the necessary microbial threshold for infection as an outcome. The 10 ⁵ threshold is held as the standard in the definition of infection in the urinary tract. A threshold of 10 ⁴ bacteria is considered the defining limit of bacteria in bronchoalveolar lavage samples of lung secretions in the critical care unit patient. There must be a definable limit of Clostridioides difficile spores that are ingested to create the enterocolitis syndrome. While there is some biological variability around the 10 ⁵ threshold, the supporting research has clearly defined that there is a limit to the magnitude of contamination that can be handled without resultant infection by the host at each anatomic site ( Fig. 2 ).

Microbial virulence

Bacteria have individual profiles of virulence characteristics. Virulence factors include structural components of the bacterial cell (e.g., endotoxin), secreted bacterial protein products that damage tissues or specifically retard phagocytic cell function (e.g., toxins), and antimicrobial resistance factors that impair the effectiveness of treatment strategies. The presence of lipid A in gram-negative endotoxin, potent exotoxins produced by Clostridium perfringens , superantigens from Streptococcus pyogenes, and coagulase from Staphylococcus aureus means that selected bacteria pose a greater infectious risk. When bacterial contaminants possess these unique virulence factors, it means that fewer microbes are required to cause clinical infection than the traditional view of 10 ⁵ bacteria per gram.

Conversely, many bacterial species lack potent virulence factors but still pose infection risks. Pseudomonas aeruginosa, Staphylococcus epidermidis, Enterococcus spp., and Candida albicans are microbes that demonstrate poor virulence characteristics in experimental models but have proven to be very real pathogens in the clinical setting. They share the capacity to resist the antimicrobial activity of many drugs and emerge as major pathogens in the immunosuppressed critical care patient when prior treatment has eliminated the more susceptible common bacterial pathogens. Intrinsically low virulence organisms in terms of structural or secreted protein products can assume major importance in critically ill patients because of resistance. In the critical care unit, virtually any microbe can assume a role as a pathogen when competing microflora have been eliminated and the host response is suppressed.

Adjuvant factors

At the end of his spectacular scientific career, Pasteur is alleged to have said, “The microbe is nothing, the terrain is everything.” While bordering on hyperbole, the comment by Pasteur emphasizes the importance of the local environment surrounding the bacterial contaminant in the trauma patient as being paramount in determining infection as an outcome. The presence of hemoglobin and hematoma in the traumatic wound or surgical site provides a rich source of nutrients and critical amounts of iron to promote microbial growth. Necrotic tissue provides a haven for bacterial proliferation that is poorly accessible by host phagocytic cells.

Foreign bodies assume importance as adjuvant variables in the development of infections in all surgical patients, but especially following injury. Traumatic wounds may contain foreign debris from the injury process, or fecal material from colonic disruption that is laden with microbes that promote infection. The surgical site will have suture material that is well documented to lower the required bacterial inoculums to cause infection. Endotracheal tubes, urinary catheters, drains, and indwelling intravascular devices that are necessary components of patient management may provide foreign body surfaces for bacteria to colonize and become the nidus for infection. The foreign body surface retards the phagocytic function of the host but also provides a surface for biofilm production as a unique virulence expression of bacteria to promote infection.

Not only do indwelling devices compromise host functions, but they may directly damage the nonspecific primary epithelial and endothelial barriers of the patient. Endotracheal tubes defeat the normal barrier of the glottis and provide a direct conduit from the external environment into the lower airway passages. Foley catheters not only create an avenue for microbial access but erode the protective transitional cell lining of the bladder. Similarly, the indwelling intravascular device creates access for treatment, but clot and endothelial damage at the site of entrance must have a role in infection.