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August 2000, Volume 50, Issue 8

Original Article

Bacteriology of Surgical Site Infections and Antibiotic Susceptibility Pattern of the Isolates at a Tertiary Care Hospital in Karachi

Abid Mahmood  ( Department of Pathology, PNS Shifa, Karachi. )


Objective: To study post surgical infections and sensitivity of the isolates so that recommendations can be made for their prevention and empirical antibiotic treatment.
Setting: Swabs/pus specimens from the patients developing surgical site infections at PNS Shifa, Karachi were processed in the Department of Pathology during January, 1998 to September, 1999.
Methods: One hundred and twenty-nine swabs/pus specimens from various types of surgical sites suspected to be infected on clinical grounds were processed, by standard methods and antibiotic susceptibility testing of all the isolates was done by using Modified Kirby Baur disc diffusion technique.
Results: Of the one hundred and fifty-three organisms isolated, the most common was Staphylococcus aureus (50.32%), followed by Pseudomonas aeruginosa (16.3%), Escherichia coli (14.37%), Klebsiella pneumoniae (11.76%), miscellaneous gram negative rods (5.88%) and Streptococcus pyogenes (1.30%). About 50% of the Staphylococcus aureus isolates were found to be methicillin resistant. In case of Pseudomonas aerüginosa and Escherichia coli more than 60% of the isolates were found resistant to Gentamicin. The resistance to third generation cephalosporins and the quinolone ciprofloxacin was also quite high. Other isolates also showed a very high level of antibiotic resistance.
Conclusion: In addition to the economic burden for antibiotic treatment, such infections for multi-resistant organisms are a serious threat to our surgical patients. To prevent these happenings, there is art urgent need to adopt basic principles of asepsis and sterilisation and to make judicious use of prophylactic and therapeutic antibiotics (JPMA 50:256, 2000).


Surgical site infections (SSI) are the third most frequently reported nosocomial infections accounting for 14-16% of all the infections in hospitalised patients. Among surgical patients SSI are the most common nosocomial infections1. These remain a complication of surgical procedures resulting in increased morbidity, mortality and cost2. The risk of developing a surgical site infection depends upon the balance between factors determining the number of bacteria contaminating the site and the factors determining the resistance of the site against infection3,4.
One of the major problems faced by the surgeons these days is to deal with the post surgical infections, as most of these are being caused by multiple resistant bacteria5. Gram positive cocci and Gram negative bacilli are being implicated in most of such cases5,6. The problem of infected surgical sites can only be tackled properly if all these are examined bateriologically and feed back given to the surgeons well in time, so that they can treat these with appropriate antibiotics7. Not only this but, the microbiologist should provide them the guidance regarding proper use of prophylactic antibiotics. But, according to the cure”. prevention of surgical rate infections by adopting basic principles of asepsis is the key to the solution of this problem.

Material and Methods

One hundred and twenty nine wound swabs/pus specimens collected from patients developing surgical site infections during a period from January, 1998 to September, 1999 were included in the study. Most of our patients were young males (n=80). Rest were females (n=38) and children (n=1 I). The age range was between 4-65 years and had undergone different kinds of surgery including general surgery (n70), gynecological/obstetric surgery (n48) and orthopedic surgery (n1 I). Pus swabs/specimens were collected from infected surgical sites by standard technique using commercially available sterile stick swabs. The specimens were immediately transported to the Department of Pathology, PNS Shifa for bacteriological study. All the specimens were inoculated onto blood and MacConkey’s agar within two hours of collection. The agar plates were incubated at 37°C aerobically and were examined for the presence of any growth after 24 hours. Those plates showing no growth were incubated for another 24 hours. The isolates were identified by colonial morphology, Gram’s stain and conventional biochemical tests, based upon methods of Cowan and Steel8 and also by using API 20 E galleries (System Montalieu Vercieu, France) for enterobacteriaceae. Antibiotic susceptibility pattern of the isolates was studied using Kirby Baur method9. Mueller Hinton agar (Difco) was used for antibiotic susceptibility testing. Staphylococcus aureus ATCC 25932, Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were included as control strains.


One hundred and fifty three organisms were isolated from the 129 specimens processed. One hundred and five specimens yielded growth of single organism while two isolates were present in rest of the twenty-four cases. The most common pathogen isolate was, Staphylococcus aureus (50.32%), followed by Pseudomonas aeruginosa (16.33%), Escherichia coli (14.37%), Klebsiella pneumoniea (11.76%), Streptococcus pyogenes (1.30%) and miscellaneous gram negative rods (5.88%) including Acinetobacter baumanni i, Proteus m irabi lis and Citrobacter diversus (Table 1).

In case of Staphylococcus aureus 49.36% of the isolates were found resistant to methicillin. Antibiotic susceptibility pattern of gram-positive cocci (Staphylococcus aureus and Streptococcus pyogenes) to other antibiotics are shown in Table 2.

In case of Pseudomonas aeruginosa only 8 isolates (32%) were
gentamicin sensitive, while all except two were sensitive to amikacin. Quite a few strains were also found resistant to pipracillin, ciprofloxacin and ceftazidime. Antibiotic susceptibility pattern of all the Gram negative rods (GNRs) studied is shown in Table 3.


Surgical site infection can be defined as the presence of pus alongwith signs of inflammation in the surgical wound margins10. Predisposing underlying conditions for surgical site infections include immunosuppression, irradiation, steroid administration, diabetes mellitus and malnutrition. The risk of infection after surgery depends upon the factors including the type and length of surgical procedure; age, underlying conditions and previous history of the patient: skill of the surgeon; diligence with which infection control procedures are applied and the type and timing of pre-operative antibiotic prophylaxis11 Most of the patients included in the study were young males with minimal predisposing factors except that six of these were diabetic. So the factors most probably operative in causing infections in our patients were related to the surgical team or surgical environment. Staphylococcus aureus is considered to be the leading pathogen in such post-surgical wound infections followed by the members of the enterobacteriaceae1,12. But in our study the Psedomonas aeruginosa was the second commonest isolate after Staphylococcus aureus. Otokwefer, TV and Datube, B13 also have found similar isolates in most of the patients included in their study.
Most of our isolates were found resistant to the commonly used antibiotics. This is a matter of great concern because treatment of such infections warrants newer and costly antibiotics. The incidence of methicillin resistant Staphylococcus aureus (MRSA) in our study in about 50%. MRSA infections cannot be treated by beta lactarnase resistant penicillins and not even by the cephalosporins14. Treatment of these infections is possible either by the Fluro­quinolones (if the isolate is found sensitive) or by the vancomycin only15. Likewise 86% of our Pseudomonas aeruginosa isolates were resistant to gentarnicin, the most commonly used aminoglycoside. But most of these isolates were found sensitive to ciprofloxacin and ceftazidime, which are thus the minimal choice to treat Pseudomonas aeruginosa infections. But an empirical treatment to be really effective against such isolates will have to include either amikacin or one of the carbapenems alone or in combination. Even the Escherichia coli, Klebsiella pneumonia and the other Gram negative isolates in our study are showing fairly high antibiotic resistance. In view of such highly resistant organisms causing wound infections in our hospital. it will become very difficult to treat these cases. So the only hope lies with prevention of such surgical site infections. To achieve this goal we will have to return to the preventive measures including fundamental principles of asepsis. Individual patient risk factors must be identified and modified whenever possible. In addition to the skin asepsis and peri-oeprative prophylactic antibiotics, care and attention to the theatre environment is also very important16. Last but not the least, surgical expertise and theatre discipline is also the essential components against surgical site infections.


We should clearly understand and identify the SSI as a problem and devise a system to track, analyze and monitor these. Hospital infection control committees should meet regularly and make recommendations at all levels for prevention of such incidents. Otherwise it will be impossible to overcome the serious issues of economic loss and high hospital morbidity and mortality caused by SSI.


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2. Bay-Nielsen M. Risk factors for surgical wound infection Ugeskr. Laeger.. 1996;158:5749-53
3. Wenzcl RP. Prevention and treatment of hospital acquired infection. In: Wyngaarden, Smith Ed, Cecil textbook of medicine. 18th ed, Philadelphia, WB Saunders Co., 988. pp.
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14.Kernodle DS. Failure of cephalosporins to prevent Staphylococcus aureus surgical wound infections. JAMA, 1990;263:961-66.
15.Sta AM, Long MN, Belcher B. Higher overall nosocomial infection rate because of methicillin resistant Staphylococcus aureus. Am. J. Infect. Control, 1993;2 1:70-74.
16.Emmerson M. A microbiologist’s view of factors contributing to infection. New Horiz, I998;6(2 Suppl): S3-10.

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