The main culprits behind postoperative infections could be the bacteria already present on the patient’s skin, according to a study conducted at the University of Washington in Seattle and published in Science Translational Medicine. The bacterial strains and the types of antibiotic resistance involved in the infections are already present in the patient before the operation and are not acquired during hospitalization.

Study Design

The study analyzed a cohort of 204 patients undergoing spinal surgery at Harborview Medical Center in Seattle between September 2019 and November 2020. It is a type of operation in which the incision is “clean” and does not involve entry into the genitourinary, digestive, or respiratory tract. The operation has a postoperative infection rate of 3%-5%. Researchers took preoperative swabs from the nose, rectum, and skin at the operation site, determining bacterial strains through genomic sequencing of the samples (metagenomics). They then compared these data with the genome of bacterial populations that had been isolated from any postoperative infections.

In 86% of cases (19 of 22 strains isolated from 14 infection cases), the bacteria causing postoperative infections are entirely similar, genetically, to those present before the operation on the patient’s body. A similar result was found for antibiotic resistances, which resembled those present in the patients’ bacterial community. A subsequent analysis of 59 postoperative infections, from a sample of 1610 operated patients, revealed that none of these infections originated from bacterial strains shared among patients. In other words, they do not seem to derive from a common source like the hospital environment.

The data indicate that most pathogens responsible for postoperative infections come from the bacterial flora of the patient, rather than from environmental contaminations during the surgical intervention. This observation is consistent with previous studies on urinary tract infections and strains of Staphylococcus aureus. The recent study from Seattle, however, is among the first to demonstrate a direct correlation between the overall composition of the skin microbiome, postoperative infections, and antibiotic resistance.

An Unexpected Discovery

The researchers also noted that the microbiome of the back is not homogeneous but varies along a gradient; gram-positive opportunistic pathogens such as Staphylococcus and Cutibacterium are more prevalent in the cervical and thoracic regions, while gram-negative and anaerobic bacteria such as Escherichia, Enterobacter, and Bacteroides are prevalent in the lumbosacral region. The only significant difference found between male and female patients was a higher prevalence of Lactobacillus in the latter.

The results have important implications for the management of surgical prophylaxis and postoperative infections. That surgical infections mainly arose from a less than perfectly sterile operating environment was almost taken for granted. If, instead, endogenous bacteria are the true culprits, then efforts to reduce the incidence of infections should focus on the patient. The study authors proposed analyzing the patient’s microbiome to personalize preoperative antibiotic prophylaxis. This approach would allow for consideration of the patient’s specific multiple resistances, thereby reducing inappropriate antibiotic use. Another recommendation is the use of antiseptics such as iodopovidone, instead of or together with chlorhexidine, against which resistant bacterial strains are developing.

In an accompanying commentary, Jack A. Gilbert, PhD, principal investigator at the UC San Diego School of Medicine, and John Alverdy, MD, professor of surgery at the University of Chicago, Chicago, hypothesized that in the future, patients could be provided with automatic kits to obtain metagenomic data and identify infection risk factors in advance. More detailed studies are needed, however, to thoroughly consider the microbial communities present in the hospital environment and establish contamination pathways. Nevertheless, they concluded, the study demonstrates that proposing generic strategies and additional levels of sterility based on the notion that postoperative infections arise from some form of external contamination may be a concept in need of revision.