Trimethoprim-Sulfamethoxazole and Azithromycin Resistance in Salmonella and Shigella: Genotype-Phenotype Concordance and Integron-MDR Association.

BACKGROUND

Rising antimicrobial resistance in Salmonella and Shigella narrows therapeutic options, especially resistance to first-line agents like trimethoprim‑sulfamethoxazole and azithromycin.

METHODS

We retrospectively analyzed 156 archived clinical isolates (Shigella n=108; Salmonella n=48) originally recovered from stool specimens collected between January 2017 and December 2020. Isolates were re-evaluated in a blinded manner using phenotypic susceptibility testing and PCR detection of dfrA1, sul1, ermB, mphA, and class 1 integron (IntI1).

RESULTS

S. sonnei was the predominant species (50.6%). Among Shigella and Salmonella isolates, resistance to trimethoprim-sulfamethoxazole and azithromycin was observed in 97.2% and 67.6% versus 16.7% and 20.8%, respectively. dfrA1 was detected in 97.2% of Shigella and 10.4% of Salmonella, while sul1 was detected in 58.3% and 14.8%, respectively. mphA was detected in 58.3% of Shigella but was absent in Salmonella, whereas ermB was detected in 0.04% and 58.3%, respectively. The PPV and NPV of ermB for phenotypic azithromycin resistance were 33.3% and 100%, respectively (κ=0.25, P<0.008). 20 Salmonella and two Shigella isolates harboring resistance-associated determinants were susceptible to azithromycin, while five Salmonella isolates with related genotypes remained susceptible to trimethoprim-sulfamethoxazole. MDR was observed in 55.6% of Shigella and 4.2% of Salmonella and was associated with IntI1 (χ2=31.99, P<0.001; Cramer's V=0.45).

CONCLUSION

Shigella and Salmonella showed distinct resistance and resistance-gene profiles. The low PPV and 100% NPV of ermB, together with genotype-phenotype discordance, indicate that molecular testing cannot replace phenotypic susceptibility testing for azithromycin resistance, although ermB may have potential rule-out value pending external validation. The association between IntI1 and MDR supports further investigation of class 1 integrons in multidrug resistance.