Antimicrobial susceptibility of Salmonella enterica serovars in a tertiary care hospital in southern India

Multidrug resistant (MDR) strains (resistant to chloramphenicol, ampicillin and co-trimoxazole) of Salmonella enterica have emerged worldwide in the last two decades1. Isolates of S. enterica with reduced susceptibility to fluoroquinolones have now appeared in the Indian subcontinent and other regions23. However, in India the degree of resistance to commonly used antibiotics such as chloramphenicol, ampicillin and co-trimoxazole in the era of quinolone resistance is not clear145. The present study was undertaken to document the change in the antibiotic susceptibility of S. enterica serovar Typhi and S. Paratyphi isolates obtained from blood culture during 2009-2011 in a tertiary care hospital in south India.

Material & Methods

All S. enterica isolates obtained from blood cultures of clinically suspected cases of enteric fever seen in Apollo Hospital, a tertiary care center in Chennai, south India, from May 2009 to June 2011 were included in the study. The study protocol was approved by the hospital ethics committee.

Antimicrobial susceptibility patterns were determined using commercial antimicrobial disks (Hi-Media, Mumbai): chloramphenicol (30 μg), nalidixic acid (30 μg), ampicillin (10 μg), azithromycin (15 μg), co-trimoxazole (1.25/23.75 μg), ciprofloxacin (5 μg), and ceftriaxone (30 μg). Antimicrobial susceptibility testing was performed in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines by Kirby-Bauer disc diffusion method6.

Minimum inhibitory concentrations (MICs) for ciprofloxacin were determined using E-test (AB Bipods, Solana, Sweden). At the time of study the MIC for ciprofloxacin was 0.5 μg/ml as per CLSI but this has subsequently been reduced to 0.0625 mg/ml. ATCC Escherichia coli 25922 strain was used for quality control7.

Results & Discussion

Of the total 322 isolates studied, 186 (57.8%) were S. Typhi and 134 (41.6%) were S. Paratyphi A, two were S. Paratyphi B. Of these isolates, 177 (55%) were sensitive to ciprofloxacin (MIC <0.25 mg/ml), 296 (91.9%) were nalidaxic acid resistant. Of the 296 nalidaxic acid resistant isolates, 278 (94%) were susceptible to ciprofloxacin (MIC <0.5 mg/ml). of the 262 isolates tested for azithromycin sensitivity, 120 (46%) were susceptible, 81 (31%) were resistant and 55 (21%) were intermediate. All 322 isolates were sensitive to ceftriaxone and chloramphenicol, 290 isolated (90%) were sensitive to ampicillin and 306 (95%) were sensitive to co-trimoxiazol (Table).

Table Sensitivity rates to various antibiotics

Enteric fever is a major public health problem in India. Various studies document S. Typhi as the commonest serovar isolated over the years8, and our study also showed 57.86 per cent isolates of serovar Typhi while 41.61 per cent were serovar Paratyphi A9.

In the last decade, there have been some reports of ciprofloxacin resistance in Salmonella10. It is believed that nalidixic acid resistance is a surrogate marker for ciprofloxacin resistance, as clinical failures have been documented in cases where ciprofloxacin has been used (based on susceptibility) for nalidixic acid resistant strains11. In our study, 13.66 per cent of isolates displayed reduced susceptibility to ciprofloxacin (MIC >0.5 μg/ml). However, as many as 94 per cent of nalidixic acid resistant isolates were ciprofloxacin sensitive by MIC testing. Kirby-Bauer disc diffusion assay using currently recommended breakpoints to ciprofloxacin may not be a reliable method, E-test should be the preferred method of choice to determine ciprofloxacin MIC1213. Routine investigation and reporting of ciprofloxacin and azitromycin MICs in patients presenting with invasive Salmonella infections, like typhoid fever have been suggested1415.

Since its introduction in 1948, chloramphenicol has been the treatment of choice for typhoid fever and remains the standard against which newer antimicrobials are compared. Treatment with chloramphenicol reduces mortality due to typhoid fever from about 20 to 1 per cent and the duration of fever from 14-28 days to 3-5 days16. However, chloramphenicol therapy has been associated with the emergence of resistance to chloramphenicol, a high relapse rate, bone marrow toxicity and high mortality rates in a recent study reported from the developing world17. Ampicillin and co-trimoxazole could be effective alternative drugs18. In our study Salmonella sp. remained sensitive to chloramphenicol, ampicillin, and co-trimoxazole (100, 90 & 95%, respectively) over the two year study period as reported earlier1019. These drugs may be preferred for treatment of enteric fever in our region.

Azithromycin has done well in clinical studies for typhoid3; however, there have been sporadic reports of azitromycin resistance20. All isolates in our study were sensitive to ceftriaxone in contrast to some studies that reported resistance to ceftriaxone2122.

A limitation of our study was that clinical outcomes were not analyzed. Quinolones may remain effective despite in vitro resistance and ceftriaxone may be associated with prolonged time to fever resolution despite in vitro sensitivity23.

In conclusion, for optimal interpretation of susceptibility, quinolone MIC is needed in cases of enteric fever where nalidixic acid is reported resistant. Azithromycin resistance is emerging. However, chloramphenicol, co-trimoxazole and ampicillin have re-emerged as valuable oral options and ceftriaxone remains a viable parenteral option for treatment of typhoid in India.