Anaerobic bacteria form a vast majority of commensal flora inhabiting different body sites and are the source of diverse infections. Among various anaerobic pathogens, the importance of GPAC as a pathogen is increasing. P. anaerobius are a member of the genus Peptostreptococcus which form an integral part of the normal microbiota of gastrointestinal tract².

P. anaerobius appear as Gram-positive cocci in pairs and short chains. On blood agar they form circular, entire edged, opaque shiny to off white colonies with wide zone of inhibition of >12mm around the SPS disc and show positive proline arylamidase test. However, it is to be noted that Peptostreptococcus stomatis, another species of genus Peptostreptococcus also produces a wide zone of inhibition around the SPS disc. The zone of inhibition is about 12-17 mm around SPS disc for P. anaerobius, whereas, P. stomatis give a wider zone of inhibition of 19-25mm^8,9. In the present study all the 30 isolates of P .anaerobius had zone size ranging between 12-16 mm. However, the isolates were further confirmed by mass spectrometry.

P. anaerobius remain a significant anaerobic pathogen with reported rates of isolation ranging from 6-7%^10,11. They are often encountered from infections of genitourinary tract, abdominal cavity, skin and soft tissue, and bone and joint infections. Generally, they are known to be involved in causation of infections below the waistline and seldom isolated from oro-dental infections^2,12. In the present study, majority (n=21, 70%) of P. anaerobius were isolated from infections involving lower extremities (Table 1). Mostly GPAC are known to be isolated from polymicrobial infections involving different anatomical sites², where as in our study, 50% of P. anaerobius isolates were obtained as monomicrobial flora.

As antimicrobial susceptibility testing is not performed routinely in many laboratories, there is paucity of susceptibility data related to GPAC. However, they are often considered to be susceptible to most of the available anti-anaerobic drugs^2,13. Agar dilution is the gold standard and recommended method for determining MIC among anaerobic bacteria. In this, suspension containing a defined number of bacterial cells are spotted on appropriate growth medium with different concentrations of the antibiotic. The MIC is defined as the lowest concentration of the antibiotic that prevents visible growth of the microorganism in the growth medium.

Metronidazole remains the empirical drug of choice for most of the anaerobic infections. In the present study, all the strains of P. anaerobius have shown excellent activity against metronidazole. Similar findings were reported in other studies^10,11,12,14. However, few studies have shown varying rates of metronidazole resistance. Baquero F et al.¹⁵,  Lee K et al.¹⁶ and Meyer L et al.¹⁷ have reported resistance rates of 5.5%, 7.4% and 41% respectively against metronidazole among Peptostreptococcus spp.

Clindamycin, a lincosamide antibiotic has good spectrum of activity against most of the Gram-positive aerobic and anaerobic organisms and acts by inhibiting bacterial protein synthesis^4,18. There are reports of emerging resistance against clindamycin among anaerobes, which could be due to modification of the target site¹⁰. Studies conducted by Brazier J et al.¹⁰, Wybo I et al.¹⁴ and Koeth LM et al.¹¹ have reported clindamycin resistance rates varying from 1.1% to 5%. Other studies have shown higher rates of resistance of 19-25% to clindamycin^15,16,17,19. Remarkably, in this study 53.3% (n=16) of P. anaerobius were resistant to clindamycin which is much higher than the earlier reports and is a point of concern.

Anaerobes develop resistance to b-lactam antibiotics by b-lactamase enzymes including penicillinases and cephalosporinases, reduced permeability through alteration in porin channels and by low affinity of penicillin binding proteins. Peptostreptococcus spp. are known to be susceptible to b- lactam drugs like penicillin G¹⁵. In our study, we found all the isolates susceptible to penicillin with low MIC₉₀. However, there are reports of penicillin resistance among GPAC in literature varying from 4% to 29%^{10, 14, 15-17}. It was also observed in the present study that, another b-lactam drug, cefoxitin had excellent activity against all P. anaerobius strains which is in agreement with the previous reports^{16, 20}. Although, Könönen E et al.¹² and Baquero F et al.¹⁵ have reported cefoxitin resistant Peptostreptococcus spp. in their studies. None of our isolates were b-lactamase producers which is concordant with other studies^{10, 12, 14}.

Chloramphenicol, a bacteriostatic agent, known to be active against most of the anaerobic bacteria has also shown good activity against all the P. anaerobius strains in our setup, in line with the findings of Lee K et al¹⁶. The development of resistance to chloramphenicol is rare, however Wybo I et al.¹⁴ have reported 3% of GPAC being resistant to chloramphenicol.

It is necessary that the anaerobic laboratory facilities be optimally utilized and susceptibility testing be performed for the frequently isolated anaerobic pathogens. Awareness of the emerging resistance among the anaerobic pathogens has to be created so as to provide appropriate antimicrobial therapy for better patient care.