Q & A

March 2005

Richard A. Savage, MD, Editor

Q.  Our microbiology laboratory reports wound culture results in a graded fashion to indicate the amount of growth on the plates as 1+, 2+, 3+, etc. Clinicians often use this information to decide whether antibiotic treatment is necessary. Is there any evidence to correlate this grading system with the likelihood of infection?

A.  Many studies have documented a correlation between the amount of bacteria in a wound and the likelihood of infection as evidenced by delayed wound healing. A recent review of wound microbiology and management by Bowler, et al provides a thorough discussion of this subject.¹

Because the majority of wound surfaces are highly contaminated with a variety of exogenous microorganisms from adjacent skin, mucosal surfaces, and the external environment, culture samples should be taken only when infection is suspected. Key clinical signs of wound infection include increased fluid drainage or purulence, or pus, redness at the wound edge margins, secondary soft tissue cellulitis adjacent to the wound, and increased pain at the site. Patients with deep tissue wound infection usually also have a fever. Clinicians should only submit wound samples from patients experiencing one or more signs of infection because indiscriminate submission of a wound specimen, especially from a superficial site, may provide useless information that leads to unnecessary antibiotic treatment.

The significance of microorganisms in wounds is also predicated on collecting material from the site using a proper wound sampling technique. No universally accepted method exists for accurately sampling wounds. In fact, there is considerable controversy regarding the preferred method for providing the most clinically relevant information about the cause of infection.¹ There are, however, several acceptable techniques for sampling wounds, including harvesting of a deep tissue biopsy, sampling wound fluids by needle aspiration where enough volume exists, and wound swabbing. Although tissue biopsy and wound fluid aspiration are the preferred methods for collecting culture specimens, most superficial and deep wound specimens are submitted as swabs because the procedure is simple, swabs are readily available, and the technique is noninvasive. If a swab sample is taken without cleansing the wound properly and removing devitalized superficial debris, then swab culture flora may only reflect superficial skin surface contamination.

Physicians usually collect only an aerobic swab for culture. Aerobic culture alone may be clinically appropriate for superficial wounds where organisms such as Staphylococcus aureus or group A Streptococci (Streptococcus pyogenes) are the primary pathogens of interest. However, studies have shown that most deep wounds are heavily colonized by aerobic and anaerobic organisms, and, in such cases as clostridial myonecrosis, abdominal surgical wound infections, and bite wounds, it is vital that anaerobic cultures are done. Deep tissue wound biopsies and fluid aspirates should be cultured for anaerobes in these clinical situations. A separate cotton-tipped swab should be collected and submitted in pre-reduced anaerobic transport tubes, or similar specialized media or systems, to recover all anaerobic species of interest.

A primary wound specimen Gram stain should be done to document the presence of pus as well as the number and types of bacterial morphotypes present in the original specimen. As with all microbiological culture analyses, appropriate interpretation of bacterial culture growth must always be correlated to the original specimen Gram stain. The presence of mixed bacterial morphotypes in a wound Gram stain that fail to grow in aerobic culture suggests the presence of anaerobes.

Several studies show good correlation between the bacteria isolated from superficial wound surfaces and that found in deeper tissue. The wound microbial load has been correlated with not only the development of infection, but also wound healing. Robson and Heggers have shown, using wound tissue biopsies, that a microbial load of >10⁵ CFU/g of tissue is diagnostic of acute and chronic wound infection.^2,3 Another quantitative tissue culture study of complex extremity wounds showed that a critical level of >10⁴ CFU/g of tissue had to be exceeded for infection to occur.4 Bendy et al used superficial wound swab samples to show that healing of decubitus ulcers occurred only when the bacterial load of wound fluid was <106 CFU/ mL.5 Burn wounds have been the most thoroughly studied with regard to quantitative tissue cultures results versus those from swabs. Good correlation has been found between the results of a superficial semiquantitative surface swab count (1+ to 4+) and a fully quantitative biopsy count in burn wounds with 1+ growth from a swab correlating with a tissue count of 10² to 10³ CFU/g and 4+ correlating with a tissue count of 10⁷ CFU/g tissue.⁶ Another study also showed that a growth of 30 CFU from a burn wound surface swab correlated well with a bacterial load of >10⁵ CFU/g of tissue.⁷ Comparisons of superficial and deep tissue cultures have also been reported from infected leg ulcers and similarly showed excellent correlation between these two methods.^8,9

More research needs to be done to correlate the quantitative microbial load of specific bacteria alone and as part of a microbial mix to determine the clinical significance of specific isolates or combinations of isolates in causing wound infection. To date, studies have only been done to show that beta-hemolytic streptococcus (group A) is capable of causing infection at microbial tissue loads that are significantly lower than 10⁵ CFU/mL.¹⁰ Until additional studies are done, it will remain difficult in polymicrobial wound infections to clinically distinguish causative and commensal species.

References

1. Bowler PG, Duerden BI, Armstrong DG. Wound microbiology and associated approaches to wound management. Clin Microbiol Rev. 2001;14:244-269.
2. Robson MC, Heggers JP. Bacterial quantification of open wounds. Mil Med. 1969;134:19-24.
3. Robson MC. Wound infection: a failure of wound healing caused by an imbalance of bacteria. Surg Clin North Am. 1997;77:637-650.
4. Breidenbach WC, Trager S. Quantitative culture technique and infection in complex wounds of the extremities closed with free flaps. Plast Reconstr Surg. 1995;95:860-865.
5. Bendy RH Jr., Nuccio PA, Wolfe E, et al. Relationship of quantitative wound bacterial counts to healing of decubiti: effect of topical gentamicin. Antimicrob Agents Chemother. 1964;10:147-155.
6. Thomson PD, Smith DJ Jr. What is infection? Am J Surg. 1994;167:7S-11S.
7. Lawrence JC. The bacteriology of burns. J Hosp Infect. 1985;6:3-17.
8. Bowler PG, Davies BJ. The microbiology of infected and noninfected leg ulcers. Int J Dermatol. 1999;38:573-578.
9. Sapico FL, Witte JL, Canawati HN, et al. The infected foot of the diabetic patient: quantitative microbiology and analysis of clinical features. Rev Infect Dis. 1984; 6: S171-S176.
10. Robson MC, Heggers JP. Delayed wound closures based on bacterial counts. J Surg Oncol. 1970;2:379-383.
    

Deirdre Church, MD, PhD
Division Head, Microbiology
Calgary Laboratory Services
Calgary, Alberta Canada
Member, CAP Microbiology Resource Committee