Antimicrobial susceptibility of Salmonella Typhi in India

Background: Typhoid fever continues to remain a major public health problem, especially in regions such as Gulbarga, due to poor sanitation and personal hygiene. Gulbarga region is often prone to enteric fever outbreaks and is an endemic region of typhoid fever. Enteric fever caused by Salmonella Typhi has not been adequately explored in this region. Methodology: A total of 95 isolates of S. Typhi collected from different clinical and environmental sources were tested for antimicrobial susceptibility according to the CLSI guidelines. MIC of resistant isolates to various antibiotics was performed by agar dilution method. Results: Of the total isolates studied, 10% were found to be multidrug resistant (MDR) (defined as resistance to ampicillin, chloramphenicol and co-trimoxazole). There was a decrease in the susceptibility to ciprofloxacin of S. Typhi with MIC showing an upward trend (0.1254μg/mL). Concurrently, there has been an increase in the number of isolates sensitive to all antibiotics except nalidixic acid. Conclusion: MDR S. Typhi continues to be an important public health issue in Gulbarga. Presence of quinolone resistance and associated low-level ciprofloxacin resistance is a concern and requires further study.


Introduction
Enteric fever is a global public health problem.Almost 80% of the cases and deaths are in Asia and the rest occur mostly in Africa and Latin America [1].It is estimated that there are 22 million new cases of enteric fever annually, with 200,000 deaths [2].Regions with the highest incidence of enteric fever ( > 100 cases per 100,000 persons per year) are South Central Asia and Southeast Asia.Regions of moderate incidence (10-100 cases per 100,000 persons per year) include the rest of Asia, Africa, Latin America and the Caribbean and Oceania, except for Australia and New Zealand.In Delhi, India, the incidence of enteric fever is 9.8 cases per 1,000 person-years [3].Salmonella enterica serovar Typhi and Paratyphi A are the predominant types of etiological agents responsible for enteric fever in India, particularly during summer [4].
The emergence of antimicrobial resistance, especially the multidrug resistance to ampicillin, chloramphenicol and co-trimoxazole, has further complicated the treatment and management of enteric fever [4,5].In India, antibiotic resistance among S. Typhi has been reported since 1960, and the first outbreak of multidrug resistant S. Typhi (MDRST) was reported in Calicut [6].Since then MDRST has appeared throughout the world, especially in South America, the Indian subcontinent, Africa and Southeast Asia [5].The incidence of multidrug resistant (MDR) S. Typhi has been reported to be as high as 60% but then declined in Pune (1999), Nagpur (2001), Delhi (2004) and Calcutta (2000) [7][8][9][10].However, resurgence of resistant strains in Ludhiana in 2002 is of concern [11].A study of imported strains that was based in the United States [12] noted an increase in the number of MDR and nalidixic acid resistant S. Typhi globally (NARST), although all isolates remained sensitive to ciprofloxacin and ceftriaxone.In Bangladesh there has been a reported decrease in MDR isolates with no corresponding increase in sensitive strains [13].For ciprofloxacin there has been an increase in MIC strains imported into the United Kingdom [14], Bangladesh [15], and India [16,17,18].These observations with variations in the sensitivity patterns reported for S. Typhi and S. Paratyphi A stress the significance of continuous monitoring of antibiotic sensitivity patterns to provide suitable guidelines for treatment.Gulbarga is a socio-economically deprived region where both personal and community hygiene are minimal.According to records of the public and private hospitals, enteric fever is a major infectious disease occurring at high fluctuating incidences.Since there are no reports on current sensitivity patterns of S. Typhi isolates from this region, this study assesses the multidrug resistance among S. Typhi isolates with emphasis on susceptibility to ciprofloxacin.

Bacterial Culture
The study included 84 S. Typhi isolates from blood cultures of patients suffering from suspected typhoid fever who attended the outpatient clinics or were admitted in the private and government hospitals of the region during August 2006 to September 2007.Eleven additional environmental isolates were obtained from sewage, water and food.A total of 1,200 blood samples and 50 environmental samples were screened for S.Typhi.
Bile salt broth (broth culture) [19] and streptokinase broth (clot culture) [20] blood samples were used for enrichment as well as Selenite F-broth for environmental samples.The enriched samples after visible turbidity were streaked on Mac-Conkey, XLD and Wilson Blair media.The isolates producing characteristic colonies were identified by conventional biochemical tests using API20E and confirmed by agglutination with Salmonella O9, Vi specific and Hd antisera procured from King Institute of Preventive Medicine Guindy, Chennai.
MICs of isolates resistant to chloramphenicol, ampicillin and nalidixic acid were determined by agar dilution test [22] using purified antibiotic powders (Himedia Laboratories, Mumbai).MIC of ciprofloxacin were determined for only 20 randomly selected nalidixic acid resistant isolates.Isolates resistant to ampicillin, chloramphenicol and cotrimoxazole were termed MDR.

Result
A total of 95 S. Typhi isolates were obtained from 1,250 samples included in the study, indicating an incidence rate of 7.6% with highest incidence in environmental (11 isolates) samples (22%) followed by that in blood (84 isolates) samples (7%).
Antibiogram of these isolates revealed that all the isolates of S. Typhi were sensitive to imipenem.Highest resistance was observed against nalidixic acid (31.57%) closely followed by ampicillin (29.47%) and chloramphenicol (28.42%).The S. Typhi isolates showed low-level resistance against the majority of the remaining antibiotics (Table1).
Approximately 10% (N = 9) of the isolates were resistant to multiple antibiotics.Of the antibiotics tested, 40% (N = 38) of the isolates had sensitive responses.MDR isolates were mainly resistant to three antibiotics: ampicillin, chloramphenicol and cotrimoxazole.
Among the 28 ampicillin resistant S. Typhi isolates, eight isolates showed an MIC of 64 µg/ml while 16 isolates showed MIC of 128 µg/ml, and for the remaining four isolates, the MIC was 256 µg/ml.Among the 27 chloramphenicol resistant S. Typhi *Defined as resistance to ampicillin , chloramphenicol and co-trimoxazole isolates, 24 have shown MIC of 32 µg/ml while the remaining three isolates have shown MIC of 256 µg/ml.Among the 20 nalidixic acid resistant isolates chosen for ciprofloxacin MIC, 18 showed an MIC of 0.125 µg/ml and two had MIC of 1 µg/ml and 4µg/ml (Table 2).

Discussion
Due to a combination of factors including poor sanitation and health care infrastructure, typhoid fever remains a major public health problem in most resource-poor countries such as India.This is the first report of S. Typhi antimicrobial susceptibility from Gulbarga.According to public and private hospital records, enteric fever is a major infectious disease occurring at high fluctuating incidences in this region.
The data presented in our study highlights that MDR, although small, exists in this region.The presence of MDR (i.e resistance to ampicillin, chloramphenicol and co-trimoxazole) was 10% for the year 2006-2007.This finding is in accordance with recent reports from some regions where the incidence of MDR S. Typhi isolates appeared to have decreased [23][24][25].The low frequency of MDR S. Typhi isolated is remarkable, since these drugs could once again be used for the treatment of enteric fever [10,25,26].
Nalidixic acid resistance is a marker for predicting low-level resistance to ciprofloxacin among S. Typhi and also an indicator of treatment failure to ciprofloxacin [10,15,25].Hence it is now recommended by CLSI (previously NCCLS) that all S. Typhi isolates should be screened for nalidixic acid resistance along with ciprofloxacin [22].Any isolate that shows resistance to nalidixic acid should be reported as intermediately susceptible to ciprofloxacin [18].Such strains have been found to be endemic in different parts of the world including India [26].In our study nalidixic acid resistance was observed in 31.5% and was associated with increase in MIC to ciprofloxacin.
In conclusion, the findings of the present study indicated that first-line antibiotics might be an effective component in the treatment of enteric fever.Also, increasing resistance to quinolones is alarming and of particular concern is the rise in MIC levels to ciprofloxacin.

Table 1 .
Number of S. Typhi isolates resistant to antibiotics by disc diffusion method.

Table 2 .
MIC value of resistant S.Typhi to various antibiotics.20 Nalidixic acid resistant isolates were tested for ciprofloxacin MIC *