Understanding antimicrobial susceptibility profile of Finegoldia magna: an insight to an untrodden path

F. magna is a clinically important GPAC with high pathogenic potential. It is frequently recovered from soft tissue infections, diabetic foot infections, deep-seated abscesses, bone and joint infections [1, 2, 4,5,6]. In this study, majority of the isolations were achieved from diabetic foot infections (31%), necrotizing fasciitis (19%) and deep-seated abscesses (19%). F. magna is known to elaborate range of putative virulence factors including protein L, peptostreptococcal albumin binding protein (PAB), subtilisin-like proteinase (SufA) and F. magna adhesion Factor (FAF) in addition to production of collagenase enzyme. The collagenase production leads to breakdown of collagen which is abundantly present in skin, tendons, cartilage and bones. This results in loss of tissue integrity and breakdown of amino acids there by producing favorable environment for growth and multiplication of asaccharolytic organism like F. magna.. There are reports mentioning the ability of F. magna in producing biofilms which may in turn interfere with the targeted antimicrobial therapy [1, 12, 13]. Thus, better understanding of the bacterial properties and their virulence mechanisms would assist the clinicians in accurate treatment and management of these infections.

Performing the antimicrobial susceptibility testing of anaerobic bacteria would be an expensive affair requiring experienced laboratory staff and adequate resources which may not be feasible in all settings. The agar dilution method, broth microdilution method or gradient tests (E test, spiral gradient test) are the various methods used for determining MICs for anaerobic organisms [8]. The practice of incorporating metronidazole disk (5 µg) in routine anaerobic culture plates and further testing of those isolates (showing zone size of less than 15 mm) for aerotolerance tests can rule out the presence of facultative anaerobic bacteria [3, 8]. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) had proposed disk diffusion susceptibility testing for anaerobes, which was found to be beneficial in fast growing organisms like B. fragilis. However, in view of poor growth and varied results (in comparison to reference agar dilution method), this technique is yet to be standardized for testing GPAC [14].

Antimicrobial resistance trends among anaerobes is highly diverse and dynamic, variability being observed between species, regions and clinical set ups. Metronidazole, a 5-nitroimidazole derivative is a common and long known antimicrobial for empirical therapy. Several complex mechanisms are implicated in the development of metronidazole resistance. There are reports of increasing resistance trends towards metronidazole among B. fragilis group. In contrast, F. magna is known to be susceptible to the commonly used anti-anaerobic antimicrobials [1, 2]. However, some studies have highlighted the resistance to metronidazole among GPAC including F. magna [15, 16]. In this study, none of the isolates showed resistance to metronidazole which was in line with other studies [13, 17,18,19,20]. However, this study did not focus on analyzing the genetic mechanisms of drug resistance.

Clindamycin is another empirical drug commonly used in treatment of anaerobic infections [14]. Variable rates of clindamycin resistance have been mentioned in literature ranging between 3 and 51% [5, 6, 15, 18, 21, 22]. This high resistance rates towards clindamycin could be attributed to alteration in target site by RNA methylase and presence of erm gene [5]. Compared to other studies, we noted a decreased resistance rate (9.5%) towards clindamycin in our setup.

GPAC are generally known to be susceptible to β-lactam group, β-lactam β-lactamase inhibitors, carbapenems and cephalosporins. In the present study, one F. magna isolate was found to be resistant to penicillin (2.4%) although no β-lactamase activity was demonstrated. There are varied reports for in-vitro activity of β-lactams among anaerobic bacteria with high rates of resistance being depicted in B. fragilis group [23]. Although good in-vitro activity of penicillin has been noted in F. magna, the resistance rates towards β-lactam group has been reported among other GPAC, particularly in P. anaerobius isolates [5]. Chloramphenicol, although not used routinely [9], has shown good susceptibility rates among anaerobic genera with an exception to study by Lee et al. [18] where two isolates of F. magna (n = 15) showed high MIC values (16–32 mg/L) and were found to be resistant. Good linezolid activity has been depicted in literature against GPAC which was concordant with our study [5, 7, 17, 24]. Multidrug resistant F. magna have been emerging and are being reported in some studies. In a study conducted by Shilnikova and Dmitrieva, a multidrug resistant F. magna was reported from mediastinal tumor showing resistance to metronidazole, ciprofloxacin, levofloxacin, penicillin G and intermediate resistance to amoxicillin-clavulanate [7].

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