High-level aminoglycoside resistance mediated by aminoglycoside-modifying enzymes among viridans streptococci:...

Area of Science:

• Microbiology
• Molecular Biology
• Antimicrobial Resistance

Background:

• Streptococcus mitis is a common bacterium found in the human microbiome.
• Multiple-drug resistance in bacteria is a growing global health concern.
• Understanding resistance mechanisms is crucial for developing effective treatments.

Purpose of the Study:

• To investigate the mechanism of high-level streptomycin and kanamycin resistance in two Streptococcus mitis strains.
• To determine if the resistance is plasmid-mediated or chromosomally encoded.
• To assess the potential for the spread of this resistance.

Main Methods:

• Phenotypic characterization of antibiotic resistance patterns.
• Biochemical assays to detect streptomycin adenylylation.
• Curing experiments using curing agents (novobiocin, acridine orange, ethidium bromide).
• Bacterial conjugation experiments to assess transferability of resistance.
• Molecular techniques including agarose gel electrophoresis and cesium chloride gradients to detect plasmids.

Main Results:

• Two strains of Streptococcus mitis exhibited high-level resistance to streptomycin and kanamycin.
• These strains adenylylated streptomycin at the 3'' position, indicating enzymatic modification.
• Resistance was not transferable via conjugation and was not eliminated by curing agents.
• No plasmid DNA was detected in the resistant strains, suggesting chromosomal location of resistance genes.
• Synergistic killing was observed with penicillin-gentamicin, but not penicillin-streptomycin or penicillin-kanamycin.

Conclusions:

• High-level streptomycin resistance in these Streptococcus mitis strains is mediated by modifying enzymes, likely encoded chromosomally.
• This represents the first report of such a mechanism in viridans streptococci.
• The findings suggest a potential for the emergence and spread of streptomycin resistance among viridans streptococci.