Anaerobes are present as normal flora of the human body. They cause infections in different parts of the body, including the respiratory tract. Due to cumbersome procedures, anaerobic bacteria often remain unidentified in clinical practice. Thus, there is a paucity of data on anaerobes causing respiratory infections. Recently, antibiotic resistance in anaerobes has been documented. Therefore, empirical therapy with known antibiotics may not be effective in most cases.
In the current scenario, Anaerobes play an important role in diseases, especially respiratory infections. Pepto streptococcus, Prevotella, Veillonella, and Parvula are commonly isolated from respiratory infections. Most of the cases have polymicrobial infections, especially with aerobic bacteria as high as 75%. These numbers are large, especially in acute exacerbations of chronic lower respiratory tract infections. Anaerobes are seen in most cases of lung abscesses and pneumonia. In our study, 50% had anaerobes, of which poly-microbial infections were seen in 70% of clinical samples. A total of 55 (67%) samples showed both aerobes and anaerobes. This is in concordance with many other studies where 68% of infections were poly-microbial [13, 14].
In the Indian scenario, a study by De et al. showed the isolation of 76 anaerobes and 122 aerobes from 100 patients with pleuropulmonary infections, e.g., empyema (n = 64), pleural effusion (n = 19) and lung abscess (n = 17). In 14% of the patients, only anaerobes were recovered, while a mixture of aerobes and anaerobes was encountered in 58%. From all cases of lung abscess, anaerobic bacteria were isolated, alone (04) or along with aerobic bacteria (13). From empyema and pleural effusion cases, 65.6% and 68.4% anaerobes were recovered, respectively. Among anaerobes, gram-negative anaerobic bacilli predominated (Prevotella malaninogenicus, Fusobacterium spp., Bacteroides spp.), followed by gram-positive anaerobic cocci (Pepto streptococcus spp.) [15]. Similar findings were also observed in other studies [16, 17].
There are a few cases reports from India on anaerobes causing chronic pneumonitis caused by Fusobacterium, Veillonella, and Prevotella (Bacillus) melanogenicus [18, 19].
Recently, resistance to metronidazole has emerged. In Bacteroides species, 15% resistance was seen in Western countries [20], and higher rates, up to 30% from a few Asian regions [21, 22], were observed. In India, metronidazole resistance varied from 7–52% [23, 24]. In our study, 20% resistance was observed. Metronidazole resistance is also emerging in other genera, namely, Prevotella and Veillonella [25]. In our study, 32% and 18% were observed in Prevotella and Veillonella, respectively. Significantly higher resistance up to 28% was seen in gram-positive anaerobes [26, 27]. In India, only 6.8% were resistant to metronidazole [23]. In our study, 42% of Bifidobacterium were resistant, but due to the smaller number of isolates (7), this cannot be commented upon.
The emergence of metronidazole resistance has led to the use of Carbapenems for the treatment of anaerobes. Carbapenem resistance has been observed to range from 1-9.6% in Western countries. East Asian literature shows 9–15% imipenem resistance across species [8]. In India, 0.6% resistance was seen. In Pakistan, 24.1% imipenem resistance was observed in metronidazole-resistant strains [28]. However, all our isolates were sensitive to meropenem. This could be because we used meropenem instead of imipenem, which was used in many earlier studies.
Resistance to cefoxitin is also on the rise. 17.2% and 35.3% were observed [23, 29]. A higher resistance, 48.2%, was observed in Bacteroides species [23]. However, in the present study, only 12% of Prevotella and 9% of Veillonella were observed. Among gram-positive anaerobes, 4% and 6% resistance of Bifidobacterium and Parvimonas, respectively, was observed. All isolates of Bacteroides were sensitive to cefoxitin.
Over the last twenty years, resistance to clindamycin has increased by 32.4% worldwide, as seen in the study by Nagy [29]. In India, overall resistance was 42.6% higher in Bacteroides (53.6%). Bacteroides fragilis showed a resistance of 46.8% [23], which is like many other studies [29, 30]. In our study, 20–69% resistance was observed in gram-negative anaerobes, with a higher percentage in Bacteroides sps (60%) and up to 16% in gram-positive anaerobes.
All our isolates were sensitive to chloramphenicol, but more than half were clustering around the breakpoints of 8 − 4 µg/L. A case of MIC creep over time may pose a threat [23]. However, in our study, resistance to chloramphenicol was seen in Veillonella (21%), Prevotella (6.5%), and Parvimonas (13%), with breakpoints of 32 µg/L. This is surprising, as chloramphenicol is not commonly used in our hospital setup.
There are a few limitations to the study. All our isolates were respiratory. We did not look for resistance genes in the isolates, and some of the genera had few isolates and thus cannot be generalized for that genus.