Antimicrobial Susceptibility in Anaerobic Bacteria

Anaerobic bacteria are integral components of the human microbiota and common agents of soft-tissue, intra-abdominal and bloodstream infections. Historically, empirical therapy with metronidazole and β-lactam–β-lactamase-inhibitor combinations provided reliable coverage, but the global rise in resistance demands renewed attention. Cultivation and identification of anaerobes remain challenging owing to oxygen sensitivity and slow growth, which can delay targeted therapy. Standardised antimicrobial susceptibility testing methods—agar dilution, broth microdilution and gradient diffusion—are now endorsed by international committees but are not universally available. Mechanistically, resistance to β-lactams is largely mediated by diverse β-lactamases; metronidazole resistance arises through reduction of the nitro group by Nim-type enzymes or inactivation pathways; clindamycin resistance is driven by Erm-type rRNA methylases; and carbapenem resistance emerges via chromosomal activation of metallo-β-lactamases such as CfiA. These factors have significant implications for antibiotic stewardship, empirical regimen selection and the development of novel diagnostics. Increasingly, molecular and proteomic approaches are being integrated with routine workflows to detect resistance determinants rapidly. The interplay between commensal reservoirs of resistance genes and pathogenic anaerobes underscores the importance of surveillance on both community and healthcare-associated fronts to guide rational therapeutic strategies and curb the spread of multidrug‐resistant anaerobic pathogens.

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Antimicrobial Susceptibility in Anaerobic Bacteria publication trend

The graph below shows the total number of articles in antimicrobial susceptibility in anaerobic bacteria across all publications each year (not limited to Nature Index journals).

Technical terms

Anaerobic bacteria: Microorganisms that grow in the absence of oxygen and often require specialised culture conditions.

Minimum inhibitory concentration (MIC): The lowest antibiotic concentration that prevents visible microbial growth under defined conditions.

β-lactamase: An enzyme produced by bacteria that hydrolyses the β-lactam ring of penicillins and cephalosporins, rendering them inactive.

Nim proteins: Nitroreductase enzymes encoded by nim genes that reduce nitroimidazole antibiotics, leading to metronidazole resistance.

Erm methylases: Ribosomal RNA methylating enzymes that confer resistance to macrolide-lincosamide-streptogramin antibiotics such as clindamycin.

Carbapenemase: A β-lactamase variant capable of hydrolysing carbapenem antibiotics, often associated with high‐level resistance.

References

1. Anaerobic Gram-Negative Bacteria: Role as a Reservoir of Antibiotic Resistance. Antibiotics (2023).
2. Proteomic analysis of metronidazole resistance in the human facultative pathogen Bacteroides fragilis. Frontiers in Microbiology (2023).
3. Recent Trends in Antimicrobial Resistance among Anaerobic Clinical Isolates. Microorganisms (2023).
4. Identification and Antimicrobial Susceptibility Testing of Anaerobic Bacteria: Rubik’s Cube of Clinical Microbiology?. Antibiotics (2017).
5. Antimicrobial susceptibility against metronidazole and carbapenem in clinical anaerobic isolates from Pakistan. Antimicrobial Resistance & Infection Control (2019).