Postoperative wound infections, despite being largely preventable, remain one of the most frequent health-care-associated infections worldwide and contribute substantially to patient morbidity, mortality, and additional costs. They represent a particularly important burden in low- and middle-income countries, where surgical site infections (SSI) have been reported as among the most frequent healthcare-associated infections (HCAI), contributing a third of all HCAI. With more than 10% of operated patients developing SSI in low- and middle-income countries, these infections are associated with increased burden on both patients and healthcare systems.(1,2) The spectrum of causative microorganisms varies with the nature of surgery, local epidemiology, and institutional infection control practices. According to a WHO report on HCAI, Gram-negative rods were isolated in a large proportion of SSIs, while Staphylococcus aureus was the most frequent cause of both SSI and bloodstream infection. Importantly, antimicrobial resistance data were limited, but among eight studies reporting methicillin resistance in S. aureus, as many as 54.5% (158/290) of isolates were methicillin-resistant. The ever-increasing burden of multidrug-resistant organisms, particularly methicillin-resistant Staphylococcus aureus (MRSA), extended-spectrum β-lactamase (ESBL)-producing Enterobacterales, AmpC β-lactamase producers, and carbapenem-resistant Gram-negative bacilli, has further complicated the management of postoperative wound infections by limiting effective therapeutic options. Knowledge of local bacterial epidemiology and antimicrobial susceptibility patterns is therefore essential for selecting appropriate empirical therapy, strengthening antimicrobial stewardship programmes, and developing institution-specific antibiotic policies.(1,3,4) Although several studies have described the bacteriology of postoperative wound infections, considerable geographical and institutional variation exists in the distribution of pathogens and antimicrobial resistance patterns. Periodic microbiological surveillance is an important component of surgical quality, hospital infection control, and patient safety programmes.(1–4) The present study was undertaken to determine the microbiological profile of postoperative wound infections in a tertiary care teaching hospital and to evaluate the antimicrobial resistance patterns of the bacterial pathogens isolated from these infections.

Study design and setting A prospective observational study was conducted in the Department of Microbiology of a rural tertiary care teaching hospital in Kolar, Karnataka, India, from June 2009 to May 2013. The hospital provides comprehensive surgical services to both urban and rural populations. Study population All postoperative wound specimens received in the Department of Microbiology from the departments undertaking Surgical procedures and carrying out post-operative follow-up during the study period were included. Each specimen represented a patient with clinically suspected postoperative wound infection referred for microbiological evaluation. The objective of the study was microbiological surveillance of postoperative wound infections. Samples from patients with perforation peritonitis and infection present at the time of surgery were also included because postoperative wound cultures from these patients formed part of the institutional surveillance programme. Clinical diagnosis Postoperative wound infection was diagnosed by the treating surgeon based on local signs of inflammation, purulent discharge from the surgical wound, wound dehiscence, or delayed wound healing. Specimen collection Pus aspirates or wound swabs were collected aseptically from clinically suspected postoperative wound infections before initiation of appropriate antimicrobial therapy whenever feasible. Specimens were transported immediately to the microbiology laboratory for processing.(5) Microbiological processing Direct Gram staining was performed for all specimens, while Ziehl–Neelsen staining was carried out whenever clinically indicated. Specimens were inoculated onto Blood agar and MacConkey agar and incubated aerobically at 37°C for 24–48 hours. Bacterial isolates were identified using colony morphology, Gram staining characteristics, and standard biochemical tests.(5) Antimicrobial susceptibility testing Antimicrobial susceptibility testing was performed using the Kirby–Bauer disc diffusion technique in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines.(6,7) Methicillin resistance among Staphylococcus aureus isolates was determined using cefoxitin disc diffusion. Extended-spectrum β-lactamase production was identified using the double-disc synergy test. Carbapenem resistance was confirmed by minimum inhibitory concentration (MIC) testing according to CLSI recommendations.(7–9) Statistical analysis Data were entered into Microsoft Excel and analysed using descriptive statistics. Continuous variables were expressed as mean and range, while categorical variables were summarised as frequencies and percentages.

A total of 808 postoperative wound specimens from patients with clinically suspected postoperative wound infections were received in the Department of Microbiology during the study period from the Departments of Surgery, Orthopaedics, Obstetrics & Gynaecology and ENT. The age of the patients ranged from less than one year to 85 years (mean age 35 years). Males constituted 539 (66.7%), giving a male-to-female ratio of approximately 2:1. Distribution according to surgical procedures: The largest number of samples were received from patients who underwent Laparotomy surgeries (340, 42.1%), followed by orthopaedic surgeries (162, 20.0%), lower segment caesarean section (95, 11.8%), appendicectomy (85, 10.5%), skin grafting (25, 3.1%), hysterectomy (25, 3.1%), other abdominal surgeries (25, 3.1%), amputations (23, 2.8%), cholecystectomy (17, 2.1%) and ENT procedures (11, 1.4%) (Table 1). Among the 260 culture-positive postoperative wound specimens, the largest proportion, 195 (75.0%), was associated with laparotomy. Of these, 167 (85.6%) were from patients with infection already present at the time of surgery, including cases involving hollow viscus perforation and other procedures involving breach of the gastrointestinal tract; these accounted for 64.2% of all culture-positive specimens. Orthopaedic surgeries accounted for 34 (13.1%) culture-positive specimens, while 31 (11.9%) were associated with lower segment caesarean section. Of the 31 culture-positive specimens following LSCS, 25 (80.6%) were associated with emergency procedures. Among all 95 LSCS-associated wound specimens, culture positivity was significantly higher following emergency procedures than elective procedures (25/41 [61.0%] vs. 6/54 [11.1%]; χ² = 24.14, df = 1, p < 0.001). Microbiological profile: Of the 808 postoperative wound specimens received, 260 (32.2%) yielded bacterial growth on culture and were included in the microbiological analysis. This represents the culture positivity rate among the specimens received. Gram-negative bacilli predominated, accounting for 215 (82.7%) isolates, whereas Gram-positive organisms constituted 45 (17.3%). Among the Gram-negative isolates, Escherichia coli was the most frequently isolated pathogen, accounting for 72 (27.7%) isolates, followed by Klebsiella spp. and Pseudomonas spp., with 48 (18.5%) isolates each. Acinetobacter spp. accounted for 20 (7.7%), Enterobacter spp. for 18 (6.9%), and Citrobacter spp. for 9 (3.5%). Non-fermenting Gram-negative bacilli constituted 68 (26.2%) of all bacterial isolates. Among these, Pseudomonas spp. accounted for 48 (70.6%), while Acinetobacter spp. accounted for 20 (29.4%). Among the Gram-positive isolates, Staphylococcus aureus accounted for 40 (15.4%) isolates, while Enterococcus spp. accounted for 5 (1.9%). Antimicrobial resistance profile: Among the 215 Gram-negative bacilli, 142 (66.0%) were extended-spectrum β-lactamase (ESBL) producers, 43 (20.0%) produced AmpC β-lactamase, and 30 (14.0%) demonstrated carbapenem resistance. Among the 40 Staphylococcus aureus isolates, 15 (37.5%) were methicillin-resistant (MRSA) and 25 (62.5%) were methicillin-sensitive (MSSA).

The WHO guidelines recognise the type of surgical procedure and degree of wound contamination as important considerations in surgical site infection prevention.(1) In the present study, laparotomy was the predominant (75.0%) surgical source of culture-positive postoperative wound specimens and among this, the major contribution (85.6%) was from patients with infection already present at the time of surgery. The predominance of laparotomy-associated specimens was similar to that reported by Negi et al. in a rural tertiary-care setting in India; however, their study was restricted to clinically diagnosed SSI cases and reported bacterial growth in 132 of 137 specimens (96.4%), compared with 32.2% among all postoperative wound specimens received in our study.(3) In our study, among LSCS-associated specimens, culture positivity was significantly higher following emergency than elective procedures (61.0% vs. 11.1%; χ² = 24.14, df = 1, p < 0.001). This association between emergency surgery and postoperative infection or SSI was also reported by earlier Indian studies. De et al., in their prospective study of 500 women undergoing LSCS, found that emergency surgery was significantly associated with SSI.(10) Similarly, Satyanarayana et al. and Mahesh et al., in prospective studies from teaching hospitals in Karnataka, reported substantially higher SSI rates following emergency than elective surgery (25.2% vs. 7.6% and 21.05% vs. 7.61%, respectively).(11,12) Although these studies assessed clinically defined SSI rather than culture positivity, their findings are consistent with the significantly higher microbiological positivity observed following emergency LSCS in the present study. The microbiological profile in the present study was characterised by a marked predominance of Gram-negative bacilli (82.7%). Escherichia coli was the most frequently isolated organism (27.7%), followed by Klebsiella spp. and Pseudomonas spp. (18.5% each). The predominance of Gram-negative organisms is similar to that reported in several Indian studies, although its magnitude and the distribution of individual pathogens have varied considerably between institutions. Kamat et al. reported Gram-negative organisms in 79% of isolates from surgical site infections in a teaching hospital in Goa, a proportion close to that observed in the present study.(13) Shah et al., in a four-year prospective study at a tertiary-care hospital in Mumbai during a period overlapping the present study, reported Gram-negative bacilli in 66% of isolates, with E. coli (22.9%), Klebsiella spp. (18.2%), Pseudomonas spp. (12.7%) and Acinetobacter spp. (6.0%) being the predominant Gram-negative pathogens.(14) Goswami NN et al. and Mahesh et al., in prospective studies conducted in comparable tertiary-care hospital settings in India, reported Gram-negative organisms in 68.9% and approximately 59% of isolates, respectively. However, Staphylococcus aureus was the single most frequent pathogen in both studies, accounting for 26.2% and 34.4% of isolates, respectively.(11,15) Negi et al. similarly reported S. aureus as the predominant pathogen (50.4%), with Gram-negative organisms accounting for approximately half of the isolates in their rural tertiary-care setting.(3) At the global level, the WHO report on the burden of endemic healthcare-associated infections noted that S. aureus was the most frequently reported pathogen in available SSI and bloodstream infection data, while Gram-negative rods accounted for a substantial proportion of SSIs, highlighting geographical and institutional variation in the microbiology of these infections.(2) Bansal et al. similarly noted that, although NNIS surveillance data had identified Gram-positive pathogens as the most frequent organisms associated with SSI, studies from Brazil, Ethiopia and other settings had demonstrated an increasing contribution of Gram-negative pathogens.(16) In the present study, the predominance of Gram-negative bacilli, particularly E. coli, may partly reflect the large representation of laparotomy-associated specimens and the substantial proportion of cases involving infection already present at the time of surgery. Non-fermenting Gram-negative bacilli (NFGNB) constituted an important component of the microbiological profile in the present study, accounting for one-fourth (68, 26.2%) of all isolates. Pseudomonas spp. constituted the majority of these isolates (48, 70.6%; 18.5% of all isolates), while Acinetobacter spp. accounted for 20 (29.4%; 7.7% of all isolates). The substantial contribution of these organisms has also been reported in earlier Indian studies. Kamat et al., in a prospective study from Goa, reported Pseudomonas aeruginosa as the most frequent isolate from surgical site infections (22.9%), followed by Acinetobacter baumannii (14.7%).(13) In the four-year prospective study by Shah et al. from Mumbai, Pseudomonas spp. and Acinetobacter spp. accounted for 12.7% and 6.0% of isolates, respectively, together comprising 18.7% of all isolates.(14) Mahesh et al. and Shahane et al. reported a high proportion of Pseudomonas aeruginosa of 26.22% and 25% respectively. Thus, NFGNB were substantial in our study, but the Pseudomonas proportion itself was within the range previously reported in Indian studies. The substantial representation of NFGNB in postoperative wound infections is clinically important because of the therapeutic challenges posed by antimicrobial resistance in these organisms. Antimicrobial resistance was a prominent feature of the Gram-negative isolates in the present study. Among the 215 Gram-negative bacilli, 142 (66.0%) were ESBL producers, 43 (20.0%) produced AmpC β-lactamase, and 30 (14.0%) demonstrated carbapenem resistance. Similar substantial burden of antimicrobial resistance has been reported in Indian studies during the same period.(3,4,9,15,16) Kamat et al. reported polyantimicrobial resistance among approximately 64% of isolates from surgical site infections, while Mahesh et al. described multidrug resistance among most isolates and reported high resistance (more than 50%) to commonly used antimicrobial agents among both Gram-positive and Gram-negative pathogens.(11,13) Shah et al., in a four-year prospective study from Mumbai overlapping the present study period, reported ESBL production in 64% of E. coli and Klebsiella spp. isolates and carbapenem resistance in 6% of Gram-negative isolates; notably, the proportion of ESBL-producing E. coli increased from 39.5% to 72.5% during their study period.(14) Although the denominators for ESBL detection differed between the two studies and therefore do not permit direct comparison of the reported proportions, the findings from both studies demonstrate the substantial burden of β-lactam resistance among Gram-negative pathogens during this period. In the present study, the identification of AmpC producers in 20% and carbapenem-resistant isolates in 14% of Gram-negative bacilli further indicates the presence of resistance mechanisms beyond ESBL production. The 2016 WHO SSI guidelines similarly identified the emergence of ESBL-producing Enterobacteriaceae as an increasing global concern, particularly because of the implications for appropriate surgical antimicrobial prophylaxis.(1) Methicillin-resistant Staphylococcus aureus (MRSA) accounted for 15 (37.5%) of the 40 S. aureus isolates in the present study. This proportion was higher than that reported in several Indian studies during the same period. Shah et al. reported methicillin resistance in 17.3% of S. aureus isolates in their four-year prospective study from Mumbai, while Negi et al. reported MRSA in 15.7% of S. aureus isolates from a rural tertiary-care hospital in Uttarakhand.(3,14) A study of SSI in referral hospitals in Mangalore, also reported substantial (9.6%) methicillin resistance among S. aureus isolates.(17) The WHO report on the burden of endemic healthcare-associated infections also documented substantial methicillin resistance among S. aureus isolates in the available SSI literature, with 158 of 290 isolates (54.5%) reported as methicillin resistant.(2) Indian studies have demonstrated considerable variation in MRSA prevalence.(3,4,9,11–13,15,16) The findings of the present study have important implications for the management of postoperative wound infections. The predominance of Gram-negative bacilli and the substantial occurrence of ESBL-, AmpC- and carbapenem-resistant isolates indicate that empirical antimicrobial therapy based solely on conventional assumptions about postoperative wound pathogens may be inadequate in this setting. At the same time, the considerable proportion of methicillin-sensitive S. aureus among the Gram-positive isolates highlights the importance of avoiding unnecessary broad-spectrum antimicrobial coverage. These findings support the need for periodic institution-specific surveillance of bacterial pathogens and antimicrobial susceptibility patterns to inform empirical treatment guidelines and surgical antibiotic policies. Integration of microbiological surveillance with antimicrobial stewardship, appropriate surgical infection-prevention practices and timely culture-guided modification of therapy may help improve treatment outcomes while limiting further selection of antimicrobial resistance.(1,2) This study has several limitations. As a laboratory-based study, the denominator comprised postoperative wound specimens submitted to the microbiology laboratory rather than the total number of surgical procedures; therefore, the culture positivity observed cannot be interpreted as the incidence of SSI. Patient-level clinical information was limited. The findings therefore primarily describe the microbiological and antimicrobial resistance profile of postoperative wound specimens received by the laboratory and may not be generalisable to all surgical patients.

This study demonstrated a marked predominance of Gram-negative bacilli among culture-positive postoperative wound specimens, with Escherichia coli being the most frequently isolated pathogen. Non-fermenting Gram-negative bacilli, particularly Pseudomonas spp. and Acinetobacter spp., constituted an important proportion of isolates. A substantial burden of antimicrobial resistance was observed, including ESBL, AmpC β-lactamase and carbapenem resistance among Gram-negative bacilli, while MRSA constituted a considerable proportion of S. aureus isolates. The significantly higher culture positivity following emergency LSCS compared with elective procedures further highlights the importance of the surgical context in interpreting postoperative wound microbiology. These findings underscore the need for continued institution-specific microbiological surveillance, culture-guided antimicrobial therapy and effective antimicrobial stewardship to inform local antibiotic policies and limit the emergence and spread of antimicrobial resistance.

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