Antimicrobial susceptibility to azithromycin among Salmonella enterica Typhi and Paratyphi A isolates from India
Richa Misra1, Kashi Nath Prasad1
1Department of Microbiology, Division Bacteriology, Sanjay Gandhi Post-graduate Institute of Medical Sciences, Lucknow 226014, India.
Abstract:
Decreased ciprofloxacin susceptibility (DCS) and multidrug resistance in typhoidal Salmonella isolates in areas of endemicity are significant therapeutic problems. Guidelines for azithromycin disc diffusion and MIC interpretive criteria for Salmonella enterica serovar Typhi were published recently by the Clinical and Laboratory Standards Institute in 2015. We investigated the antimicrobial susceptibility pattern of azithromycin in 100 isolates of Salmonella Typhi (n=80), Paratyphi A (n=18) and B (n=2) recovered from bloodstream infections from January 2013 to December 2015. Zone sizes were extrapolated against MIC values, and a scatter plot was constructed. The azithromycin MICs by Etest ranged from 2 to 16 µg ml-1, while the disc diffusion diameters were from 13 to 22 mm. We observed that the margin of the zone of inhibition around the azithromycin disc may not be very clear and therefore difficult to interpret and that there was wide variation in the zone sizes for the same MIC value in both serovars. DCS was observed in 85 % of Salmonella Typhi recovered (68/80) and in 15/18 (83.3 %) Paratyphi A isolates. Judicious use of azithromycin is advocated as an alternative oral agent in endemic areas where DCS is common.
Insights
Decreased susceptibility to azithromycin is a growing concern in typhoidal Salmonella. This study found high rates of decreased azithromycin susceptibility in Salmonella Typhi and Paratyphi A isolates, highlighting the need for careful drug use.
Area of Science:
- Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- Decreased ciprofloxacin susceptibility (DCS) and multidrug resistance in typhoidal Salmonella pose significant therapeutic challenges in endemic regions.
- Recent guidelines for azithromycin disc diffusion and MIC interpretive criteria for Salmonella enterica serovar Typhi were published in 2015.
Purpose of the Study:
- To investigate the antimicrobial susceptibility pattern of azithromycin in Salmonella Typhi, Paratyphi A, and Paratyphi B isolates from bloodstream infections.
- To evaluate the reliability of azithromycin disc diffusion testing for Salmonella isolates.
Main Methods:
- Antimicrobial susceptibility testing of 100 Salmonella isolates (80 Typhi, 18 Paratyphi A, 2 Paratyphi B) using azithromycin disc diffusion and Etest.
- Correlation of zone sizes with Minimum Inhibitory Concentration (MIC) values through scatter plot analysis.
Main Results:
- Azithromycin MICs ranged from 2 to 16 µg/mL, with disc diffusion diameters from 13 to 22 mm.
- Difficulty in interpreting azithromycin disc diffusion zones due to unclear margins and wide variation in zone sizes for equivalent MICs.
- High rates of decreased azithromycin susceptibility observed: 85% in Salmonella Typhi and 83.3% in Paratyphi A.
Conclusions:
- Azithromycin disc diffusion interpretation can be challenging for Salmonella isolates.
- High prevalence of decreased azithromycin susceptibility necessitates judicious use of azithromycin.
- Azithromycin remains a potential oral alternative in endemic areas with high rates of decreased susceptibility, but careful monitoring is crucial.
