The isolation pattern, species distribution and antibiotic susceptibility profile of coagulase negative Staphylococci: emerging opportunistic pathogens

Background: Coagulase negative staphylococci (CoNS), once considered as non-pathogenic commensal of skin and mucus membrane have emerged as important cause of health care associated infections (HCAI).  Objective: The present study was conducted with an aim to study isolation pattern, species distribution and antibiotic susceptibility profile of CoNS isolated from various clinical specimens. Method: A total of 385 CoNS isolated various clinical specimens were included in the study. Kloos and Schleifer scheme was used for speciation of the isolates. The antibiotic susceptibility testing of the isolates was performed on Mueller Hinton agar by Kirby –Bauer disc diffusion method. Results: S. epidermidis followed by S. haemolyticus and S. saprophyticus were the maximum isolated species of CoNS.  Majority CoNS isolates were resistant to routinely used antibiotics. A total of 102 (29.3%) isolates were sensitive to methicillin. Conclusion:  Isolation of CoNS can’t be overlooked as commensals nor dismissed as contaminants. S. epidermidis, S. haemolyticus and S. saprophyticus are most common species of CoNS implicated in human infections and often demonstrate low sensitivity to commonly used antibiotics. Therefore species identification and antibiotic sensitivity testing appears to be mandatory for individual assessment as no particular pattern can be predicted in any intricate situation caused by CoNS.


Results
The distribution of CoNS in different clinical specimens is shown in Figure.1 majority isolates were obtained from blood cultures (28%) followed by urine samples (21%). Figure 2 shows the species wise distribution of CoNS isolated from various clinical specimens. S. epidermidis (38.5%) followed by S. haemolyticus (28.4%) and S. saprophyticus (23.8%) were the maximum isolated species of CoNS. S. cohnii (1.1%) and S. schleiferi (0.2%) were the least isolated species of CoNS. The majority of isolates obtained from blood cultures were S. epidermidis and S. haemolyticus. Most of these blood cultures were received from neonatal intensive care unit (NICU). In urine cultures S. epidermidis and S. saprophyticus were the common isolates. S. saprophyticus was the common isolate in young women and S. epidermidis was common in patients with urinary catheters.
As shown in Table 1 maximum CoNS isolates were resistant to antibiotics like Penicillin G, cotrimaxazole, amoxicillin and erythromycin. A total of 102 (29.3%) isolates were sensitive to methicillin. Antibiotic resistance was more common in S. epidermidis as compared to other species of CoNS. In this study all isolates were sensitive to vancomycin.

Discussion
In recent years infections caused by CoNS are increasingly reported. 10,11 CoNS are now considered as one of the important etiological agents of health care associated and opportunistic infections. The increase can be attributed more to predisposing host factors like immunocompromised status, chronic illness, presence of foreign body and presence of one or other type of indwelling prostheti c medical devices rather than change in virulence of organism. 12 In our study majority of CoNS were obtained from blood cultures and most of these blood cultures were from NICU patients. In the study of Roy et al CoNS were the third most common cause of bacteremia in neonates. 11 CoNS can cause life threatening infection in high risk neonates like with poor perinatal history, premature new born, neonates with invasive producers like umbilical arterial/ venous catheterization, endotracheal intubation, jugular or subclavian central alimentation catheter etc. 4 S. epidermidis and S. haemolyticus were most common species of CoNS isolated from blood cultures. Our observation is in accordance to that of Low et al. 13 S. epidermidis is emerging as an important cause of morbidity and mortality in immunocompromised patients. It is most common cause of septicemia in leukemia and lymphoma patients. 14 CoNS are the most common bacteria colonizing skin and mucous membrane and can contaminate the blood culture if proper aseptic techniques are not followed while collection of blood. The clinical isolate of CoNS are considered as significant pathogen when it is isolated in pure culture or repeatedly isolated.
CoNS are well known cause of urinary tract infection (UTI). 15 In our study S. epidermidis and S. saprophyticus were the common isolates obtained from urine samples. Our observation is similar to that of Kumari et al, 10 Nicolle et al 16 and Leighton et al. 17 S. saprophyticus is a true urinary pathogen causing upper and lower urinary tract infection. It is the most common causative organism of cystitis in young sexually active women. 18 Urease production is most important virulence factor contributing to pathogenicity of S. saprophyticus. 19 S. saprophyticus also possess increased capacity of adherence to the uroepithelial cells. UTI due to S. epidermidis and other CoNS is usually catheter induced and it is more commonly seen in elderly patients with instrumentation or surgical intervention for urological abnormalities. 19 Blood 28% CoNS may be recovered from variety of cutaneous lesions (cyst, carbuncles, and furuncles) both alone and along with pyogenic bacteria including S. aureus and β haemolytic Streptococci. The most commonly implicated species are S. epidermidis, S. haemolyticus, S. lugdunensis and S. hominis. 20 CoNS are ubiquitous and may act as a reservoir for drug resistance gene. The widespread use of broad spectrum antibiotics in community and health care set ups has contributed to the drug resistance in Staphylococcus spp. In the present study majority of CoNS isolates demonstrated resistance to commonly used antimicrobial agents. Our observation is similar to that of Mitt et al. 21 In our study S. epidermidis demonstrated less sensitivity to antimicrobial agents as compared to other species. Multidrug resistance in S. epidermidis has strong association between prolong hospitalization especially in surgical ICU and parenteral hyperalimentation. 22 In our study only 29.3% of CoNS isolates were sensitive to methicillin. The methicillin resistance in S. aureus was first reported in 1961, at that time CoNS was considered as commensal with less or no clinical significance hence for a long period of time there was no information about methicillin resistance in CoNS. 23 Ohman et al reported methicillin resistance in 70-80% of CoNS isolates. 5 All isolates of CoNS were sensitive to vancomycin which is in accordance to the observation of Goyal et al. 3

Conclusion
Coagulase negative Staphylococci have emerged as important cause of opportunistic and health care associated infections. CoNS can't be overlooked as commensals nor dismissed as contaminants. S. epidermidis, S. haemolyticus and S. saprophyticus are most common species of CoNS implicated in human infections and often demonstrate low sensitivity to commonly used antibiotics. Therefore species identification and antibiotic sensitivity testing appears to be mandatory for individual assessment as no particular pattern can be predicted in any intricate situation caused by CoNS.