Antibiotic susceptibility study of human gut-derived facultative anaerobic bacteria

The human gastrointestinal tract is an ideal habitat for many microbes and obligate and facultative anaerobes are dominant microbes in the human-gut. The critical role of facultative anaerobes in host infections has been previously studied. The gut- microbiota is a key determinant in the defensive action of the host’s intestinal barrier, and in immunocompromised individuals, disruption of GIT homeostasis leads to gut dysbiosis. This changes the conditions which commensal microorganisms experience and can lead to a shift in microbial opportunistic pathogenicity. With few exceptions, most human pathogens are facultative anaerobes. Under oxic conditions facultative anaerobes respire oxygen and switch to anaerobic respiration or fermentation in oxygen-deprived conditions. Antibiotics are the best option to treat bacterial infections, and antimicrobial susceptibility testing is the method to determine the susceptibility of a pathogen to identify the appropriate antibiotic therapy. Generally, appropriate antibiotic concentrations for treatment are determined by AST and MIC testing under in vitro aerobic conditions. However, the possibility that the target pathogen may respond differently to antimicrobial compounds under anaerobic conditions has been neglected.

A study conducted by group of Dr. Om Prakash Sharma at NCMR-NCCS Pune reported that microorganisms isolated from human gut respond in different way to some antibiotic drugs in the presence or absence of oxygen, and the minimum inhibitory concentration values are significantly different between oxic and anoxic growth conditions. This study is highlighted in the Journal of Microbes and Infection. This was reported by analyzing 17 bacterial strains from 15 genera of facultative anaerobes isolated from the human-gut. Facultative anaerobes cause infections in anoxic parts of the human body like deep wound, gastrointestinal tract, lungs etc. Generally, antibiotic susceptibility tests for facultative anaerobes are performed under aerobic conditions due to ease of handling and rapid growth. However, variations in susceptibility of facultative anaerobes to antibiotics under aerobic and anaerobic conditions can lead to failure of antibiotic treatment. This study evaluated the susceptibility of facultative anaerobic microorganisms to antibiotics during growth under anaerobic or aerobic conditions.

A total of 17 facultative anaerobic bacterial strains from the feces of a healthy individual were selected for antibiotic susceptibility testing under aerobic and anaerobic conditions. The 17 strains belonged to 15 different genera and 3 different phyla and represent a broad phylogenetic range of organisms present in the human gastrointestinal tract. Of the tested strains, variable susceptibility to antibiotics under aerobic and anaerobic conditions was observed in 10 of 17.  As there are very few prior studies examining differential antimicrobial susceptibility under aerobic and anaerobic conditions, there is no equivalent susceptibility data for selected organisms and antibiotics combination in database for comparison.

Bacterial strains from the genera Streptococcus, Pellistega, Enterobacter, Lactococcus, Klebsiella, Bacillus and Arthrobacter showed similar resistance patterns for each antibiotic under aerobic and anaerobic conditions and did not meet the criteria for further analysis. Differential antimicrobial susceptibility was observed for at least one compound in bacterial strains from the genera Mixta, Weissella, Enterococcus, Micrococcus, Staphylococcus, Enterobacter, Bacillus (tequilensis), Kocuria, and Kluyvera. Significantly lower MIC values under aerobic conditions relative to anaerobic conditions were observed for Enterococcus faecalis and Enterobacter hormaechei subsp. steigerwaltii in the presence of cefixime. Similarly, significantly lower MIC values under aerobic conditions relative to anaerobic conditions were observed for Mixta calida, Staphylococcus hominis and Weissella cibaria when in the presence of cefpodoxime. Conversely, significantly higher MIC values in aerobic conditions relative to anaerobic conditions were observed for Kluyvera cryocrescens in the presence of ceftriaxone. The MIC of Kocuria indica was significantly higher for ceftriaxone in anaerobic conditions relative to aerobic conditions.

Researchers compared the resistance patterns of representative microbes from 15 bacterial genera isolated from the human- gastrointestinal tract against 22 different antibiotics from six classes under aerobic and anaerobic conditions. Preliminary results obtained by a disc diffusion method were verified using minimum inhibitory concentration (MIC) testing. The results demonstrated that 7 strains had a similar pattern of drug resistance under both conditions, while the remaining 10 strains had significant differences in resistance patterns between aerobic and anaerobic conditions for at least one antibiotic. The study concluded that successful antibiotic therapy requires proper analyzing of the oxygen condition of the growth environment and MIC testing of each pathogen under anaerobic and aerobic conditions.


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