Antibiotic susceptibility patterns of
common enteric pathogens in India
Mary Jesudason1 and Philippa Shanahan, SGB Amyes and CJ Thomson2
of Microbiology, Christian Medical College and Hospital, Vellore, South India
of Medical Microbiology, University of Edinburgh Medical School, Edinburgh, UK
Resistance to antibiotics by common enteric pathogens has reached
alarming levels in India. Previously susceptible organisms, responsible for some of this country's more serious
health threats such as typhoid fever and gastroenteritis are showing increasing resistance to first-choice antimicrobials.
This rise in resistance may be associated with population patterns, particularly crowded living conditions. The
availability of antibacterial drugs without prescription may also encourage their indiscriminate use and may help
to maintain high resistance levels. India's medical profession may further be contributing to the problem by prescribing
The Department of Microbiology, Christian Medical College and Hospital,
Vellore, South India, has been regularly monitoring the antibiotic susceptibility patterns of common enteric pathogens
to frequently used antibacterial agents. In addition, a study was conducted in 1989 to determine the possible reservoir
of resistant genes within local populations. We examined the incidence and type of resistance in the commensal
fecal flora of healthy individuals in Vellore and three surrounding villages. Volunteers above five years of age
were recruited who had not been treated with antibiotics during the prior two weeks. Results show that a vast majority
of the population carries commensal bacteria resistant to trimehtoprim, ampicillin or chloramphenicol. Often the
organisms isolated were resistant to all three drugs. Surprisingly, there was little difference in degrees of resistance
between villages and urban areas, though it might be assumed so, given that antibiotics are more freely available
to the urban population.1
Over the years, we have isolated S. typhimurium in substantial numbers. In 1996, we saw isolates that were multidrug resistant,
and had a cluster outbreak in one of our wards. Some of these patients suffered nosocomial infections, though spread
was contained once control measures were instituted. This strain was resistant to ampicillin , chloramphenicol,
tetracycline, furozolidone, cefazolin, gentamycin and nalidixic acid; it was sensitive to the fluorinated quinolone,
During an outbreak of gastroenteritis (GE) due to MDR S. typhimurium,2 trimethoprim
MIC was determined for 96 strains. Eighty-four (87.5%) were resistant to 10 ml/L Tp and this was HLR (MIC >1000
mg/L Tp) in 83 (86.5%). Transfer experiments showed that both trimethoprim and multidrug resistance were plasmid-mediated.
Since 1990, medical microbiologists and clinicians in India have been concerned with the emergence of multidrug
resistant S. typhi,
the pathogen for typhoid fever.3,4 This strain has shown resistance to ampicillin, chloramphenicol and co-trimoxazole,
the drugs of choice in the treatment of this disease. During March 1992, while monitoring its isolation on a monthly
basis, we recorded a record 48 of 61 S. typhi isolates (78%) as being multidrug resistant. Although 1997 saw a decrease
in their frequency, multidrug resistant S. typhi still accounts for about 30% of isolates from blood cultures. Fortunately,
during this period we continued to isolate chloramphenicol-sensitive strains. Their relative frequency appeared
to be unaffected by the epidemic of multidrug resistance. The two strains behaved as though they were epidemiologically
To study multidrug resistant S.
typhi in depth, we looked at 15 strains. In transfer studies,
we were able to demonstrate that resistance to ampicillin, chloramphenicol and co-trimoxazole was transferred 'en
block' to recipient E. coli, suggesting that resistance is mediated by an autotransferring plasmid.3 Four different
plasmid profiles were identified, and plasmid sizes ranged from between 110 and 150.6 Plasmid-mediated antibiotic
R genes were identified with specific probes in hybridization experiments.
Ciprofloxacin is the drug of choice when confronted with multidrug
resistant S. typhi.
A clinical trial, begun in 1989, showed that during 1989 and 1990, patients typically responded to ciprofloxacin
therapy after a mean of 3 days.7 MICs of this drug to multidrug resistant S.
typhi were between 0.0009 to 0.0039 mcg/ml. However, during
and after 1993, anecdotal information suggested that occasional patients with typhoid fever showed a delayed response
to this antibiotic. During this same time period, we also noticed an upward shift in MIC values in our laboratory,
to 0.25 to 0.5 mcg/ml;8 however, thus far, all strains have been in the sensitive MIC range.
typhi strains isolated in Vellore, one in 1992 and and the
remainder in 1994, showed reduced susceptibility to 4-quinolones (MIC of ciprofloxacin 0.256 mg/L). Plasmid-mediated
antibiotic R genes were identified with specific probes in hybridization experiments.The gyrA gene from codons
24 to 185, which includes the "Quinolone Resistance Determining Region," was amplified by the PCR, and
after separating the amplified stands, the DNA was sequenced. In 1994, nine isolates had a mutation in codon 83,
changing serine to phenylalanine. Also, that year one strain had a mutation in codon 87, changing aspartate to
tyrosine. Another 1994 strain had both mutations. The 1992 strain had no discernible alteration in the gyr A region,
compared to the ciprofloxacin-sensitive isolates.9
is the most common aetiologic agent of GE in Vellore. In the early 1980s, antibiotic susceptibility patterns of
the commonly encountered S. flexneri
and S. sonnei
showed resistance to ampicillin, chloramphenicol and tetracycline.10 This is still the case today. Transfer studies
have been carried out on these strains by standard methods, using E.
coli J62-2 as a recipient. Resistance was transferable and
probably plasmid-mediated. Antibiogram of transconjugants showed that SM and ampicillin resistances were linked
most commonly with the Tp/Co.11
Since June 1989, we have encountered strains of Shigella resistant
to nalidixic acid, a drug commonly used to treat shigellosis in children. Since newer quinolones are increasingly
being used to fight various infections, and cross resistance among the quinolones is known to occur, we tested
these nalidixic acid-resistant strains for susceptibility to four fluorinated quinolones: ciprofloxacin, ofloxacin,
pefloxacin and enoxacin. MIC values were determined by the Kirby Bauer disc diffusion method. Five strains of S. flexneri (serotype
2) and one strain of S. shigae
were found to be resistant to nalidixic acid, with MIC values greater than 10 mg/1; one strain had a MIC greater
than 100 mg/1. MIC values of four newer quinolones showed that all six strains were resistant to ciprofloxacin,
pefloxacin and enoxacin, but were moderately sensitive to ofloxacin. Our transfer experiments failed to demonstrate
Initially, all strains of V. cholerae
01 Eltor Ogawa isolated from feces in our laboratory were sensitive to antibiotics used to treat acute GE, including
cholera. In July 1987, however, cotramoxazole-resistant strains were observed. MIC determinations showed that the
resistance to Tp was HLR and could be transferred to recipient E.
coli, again suggesting plasmid-mediatation. The resistance
profile for the tranconjugants showed that 74% of the TpR strains studied were also resistant to chloramphenicol
and 11% to ampicillin.13
In past years, all isolates of V.
cholerae 01 were sensitive to nalidixic acid. Since October
1995, however, we have encountered strains resistant to this drug. This is rather alarming, as nalidixic acid is
the drug of choice for empirical treatment of GE, especially in children, and is also considered to be the treatment
of choice for Shigellosis. We studied the sensitivity of V.
cholerae 01 to norfloxacin, a fluoroquinolone often used
to treat GE. MIC studies confirmed all strains were sensitive to the drug.14
India, like many other developing nations, continues to face special
challenges in the face of increasing antibiotic resistance. Careful studies, increased public awareness, and sound
policies and practices will go a long way to help curb this threat to our quality of life.
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