Sobhan Nandi - Academia.edu (original) (raw)

Papers by Sobhan Nandi

Antimicrobial Agents and Chemotherapy, Oct 1, 2005

Neisseria gonorrhoeae becomes resistant to tetracycline by two major mechanisms: expression of a ... more Neisseria gonorrhoeae becomes resistant to tetracycline by two major mechanisms: expression of a plasmidencoded TetM protein and mutations in endogenous genes (chromosomally mediated resistance). Early studies by Sparling and colleagues (P. F. Sparling F. A. J. Sarubbi, and E. Blackman, J. Bacteriol. 124:740-749, 1975) demonstrated that three genes were involved in high-level chromosomally mediated tetracycline resistance (MIC of tetracycline > 2 g/ml): ery-2 (now referred to as mtrR), penB, and tet-2. While the identities of the first two genes are known, the tet-2 gene has not been identified. We cloned the tet-2 gene, which confers tetracycline resistance, from tetracycline-resistant clinical isolate N. gonorrhoeae FA6140 and show that resistance is due to a single point mutation (Val-57 to Met) in the rpsJ gene (rpsJ1) encoding ribosomal protein S10. Moreover, the identical mutation was found in six distinct tetracycline-resistant clinical isolates in which the MIC of tetracycline was >2 g/ml. Site-saturation mutagenesis of the codon for Val-57 identified two other amino acids (Leu and Gln) that conferred identical levels of resistance as the Met-57 mutation. The mutation maps to the vertex of a loop in S10 that is near the aminoacyl-tRNA site in the structure of the 30S ribosomal subunit from Thermus thermophilus, and the residue equivalent to Val-57 in T. thermophilus S10, Lys-55, is within 8 to 9 Å of bound tetracycline. These data suggest that large noncharged amino acids alter the rRNA structure near the tetracycline-binding site, leading to a lower affinity of the antibiotic.

Antimicrobial Agents and Chemotherapy, Sep 1, 2008

Antibiotic resistance in Neisseria gonorrhoeae continues to be a major concern in public health. ... more Antibiotic resistance in Neisseria gonorrhoeae continues to be a major concern in public health. Resistance of N. gonorrhoeae bacteria to penicillin G is widespread in most developed countries, which has necessitated a change to newer drugs for treatment of gonococcal infections. Recent reports indicate that resistance to these newer drugs is increasing, highlighting the need for accurate therapeutic recommendations. In some countries or communities, however, N. gonorrhoeae isolates are still susceptible to penicillin, so the use of this antibiotic for single-dose treatments of medically under-resourced patients is beneficial. In order to evaluate the adequacy and sustainability of this treatment approach, we explored the presence and prevalence of chromosomally mediated resistance determinants in N. gonorrhoeae isolates collected from 2005 to 2007 in New Caledonia. We developed two new real-time PCR assays targeting the penB and mtrR determinants, to be used together with a previously described duplex assay targeting the penA and ponA determinants. The results of this study provided evidence that neither the most-common mtrR determinants nor the most-resistance-associated penB alleles are currently circulating in New Caledonia, suggesting that penicillin should still be considered a valuable treatment strategy. Additionally, using our genotyping assay, we observed an unexpected penB genotype at a relatively high frequency that was associated with a decreased susceptibility to penicillin (average MIC, 0.15 g/ml). Sequencing revealed that this genotype corresponded to an A102S mutation in the penB gene. The molecular tools developed in this study can be used successfully for prospective epidemiological monitoring and surveillance of penicillin susceptibility.

PubMed, 2001

Objective: To estimate the incidence and risk factors of group A streptococcus (GAS) sore throat ... more Objective: To estimate the incidence and risk factors of group A streptococcus (GAS) sore throat among school-aged children living in a periurban slum area of Chandigarh, North India. Methods: A total of 536 children aged 5-15 years from 261 families identified by a systematic random selection method were enrolled in the study. Episodes of sore throat were recorded through fortnightly home visits over a one-year period. The local vernacular (Hindi) terms gala kharab (bad throat) and khansi jukam (cough and cold) were used to identify symptoms of sore throat, and throat swab specimens were collected from children who had these symptoms on the day of the home visit. Bacterial culture was carried out and the isolation of GAS was confirmed using group-A-specific antiserum. Findings: The incidences of sore throat and GAS sore throat were, respectively, 7.05 and 0.95 episodes per child-year. The incidence was higher in the following situations: among 11-year-olds, during the winter (November to January) and rainy (August) months (a bimodal peak), among children living in houses where there was no separate room for the kitchen, and in homes that included a tobacco smoker. Conclusion: The results show that the incidence of GAS sore throat was related to age, season, and indoor air pollution.

Letters in Applied Microbiology, Sep 1, 2002

To monitor the presence of various pyrogenic exotoxin genes in strains of Streptococcus pyogenes ... more To monitor the presence of various pyrogenic exotoxin genes in strains of Streptococcus pyogenes isolated in India. Methods & Results: Isolates recovered from pharyngitis (52) and rheumatic fever (RF) ⁄ rheumatic heart disease (RHD) (8) patients were analysed for the presence of toxin genes, speA, speB and speF, by PCR. The specificity of the products was confirmed by restriction enzyme digestion and Southern hybridization. Among the 60 isolates studied, the incidence of speA, speB and speF were 5(8AE3%), 56(93AE3%) and 53(88AE3%), respectively. The expression of these genes was established in representative isolates by RT-PCR. Conclusions: Comparative analysis of frequency of the speA, speB and speF genes, among pharyngitis and RF ⁄ RHD associated isolates, showed higher incidence in RF ⁄ RHD (25%, 100%,100%) as compared to pharyngitis patients (5AE8%, 92AE3%, 86AE5%), respectively. Significance of Study: The presence of the speA gene, which is usually associated with scarlet fever or toxic shock-like syndrome, within few Indian isolates may be indicative of new virulent strains circulating within the Indian community. High distribution of toxin genes among RF ⁄ RHD compared to pharyngitis isolates indicate their possible role in increased virulence.

Letters in Applied Microbiology, Jul 1, 2004

To monitor molecular heterogeneity among the clinical isolates of group A Streptococcus (GAS) fro... more To monitor molecular heterogeneity among the clinical isolates of group A Streptococcus (GAS) from north India by Vir and emm typing. GAS isolates, 31 from pharyngitis and nine from rheumatic fever (RF)/rheumatic heart disease (RHD) patients were differentiated into 16 Vir types (VT). These isolates were further discriminated into 23 emm types. Most of emm types were Vir type specific, except few (7.5%), which revealed different Vir types within same emm type. The most prevalent emm type found was emm 49 (15%) followed by 7.5% of emm 69, emm 71 and emm 75 which were different from emm type distribution reported from south India. Analysis of data revealed 40% heterogeneity by Vir typing and 57.5% by emm typing among GAS isolates which is significant in view of small number of isolates studied. SIGNIFICANCE OF IMPACT OF THE STUDY: The molecular study for the first time demonstrates different emm types prevalent and circulating in northern region of India and such data may help in selection of types for vaccine development.

Proceedings of the National Academy of Sciences of the United States of America, Apr 23, 2004

Investigative Ophthalmology & Visual Science, 2011

Bulletin of the World Health Organisation

To estimate the incidence and risk factors of group A streptococcus (GAS) sore throat among schoo... more To estimate the incidence and risk factors of group A streptococcus (GAS) sore throat among school-aged children living in a periurban slum area of Chandigarh, North India. A total of 536 children aged 5-15 years from 261 families identified by a systematic random selection method were enrolled in the study. Episodes of sore throat were recorded through fortnightly home visits over a one-year period. The local vernacular (Hindi) terms gala kharab (bad throat) and khansi jukam (cough and cold) were used to identify symptoms of sore throat, and throat swab specimens were collected from children who had these symptoms on the day of the home visit. Bacterial culture was carried out and the isolation of GAS was confirmed using group-A-specific antiserum. The incidences of sore throat and GAS sore throat were, respectively, 7.05 and 0.95 episodes per child-year. The incidence was higher in the following situations: among 11-year-olds, during the winter (November to January) and rainy (Aug...

Journal of Bacteriology, 2015

In strains of Neisseria gonorrhoeae harboring the mtr and penB determinants that decrease permeat... more In strains of Neisseria gonorrhoeae harboring the mtr and penB determinants that decrease permeation of antibiotics into the periplasm, mutation or deletion of the PilQ secretin of type IV pili increases resistance to penicillin by ∼3-fold, indicating a role for PilQ in antibiotic permeation. In this study, we examined spontaneously arising mutants with decreased susceptibility to penicillin. One class of mutants had a phenotype indistinguishable from that of a previously characterized pilQ2 mutation that interfered with the formation of SDS-resistant PilQ multimers. A second class of mutants contained frameshift mutations in genes upstream of pilQ in the pilMNOPQ operon that increased resistance to levels similar to those of the pilQ2 mutation. In-frame deletions of these genes were constructed, but only the frameshift mutations increased antibiotic resistance, suggesting that the mutations had polar effects on PilQ. Consistent with this result, titration of wild-type PilQ levels r...

The Indian journal of medical research, 2008

Group A streptococcus (GAS) causes a wide variety of life threatening diseases in developing coun... more Group A streptococcus (GAS) causes a wide variety of life threatening diseases in developing countries like India. Characterization of GAS is therefore necessary for prevention and control of the disease. Genotypic analysis of GAS is largely lacking from India, therefore an attempt was made to study the genotype distribution of north Indian GAS isolates. Sixty clinical isolates of GAS, (52 collected from pharyngitis and 8 from RF/RHD patients) were genotyped by various molecular techniques like restriction enzyme analysis (REA), ribotyping, PCR-ribotyping and random amplification of polymorphic DNA (RAPD). A few isolates were also typed by emm gene sequencing for comparison. REA using Hind III digestion differentiated the isolates into six different patterns. The same isolates were grouped into three ribotypes when analyzed for PCR - ribotyping of 16S- 23S rRNA region. However, RAPD fingerprints generated higher level of discrimination by AP4 and AP5 primers showing 12 rapdemes, fol...

Proceedings of the National Academy of Sciences, 2004

Reversing the spread of antibiotic multiresistant bacteria is hampered by ignorance of the natura... more Reversing the spread of antibiotic multiresistant bacteria is hampered by ignorance of the natural history of resistance genes, the mobile elements carrying them, and the bacterial hosts harboring them. Using traditional cultivation and cultivation-independent molecular techniques, we quantified antibiotic resistance genes and mobile elements called integrons in poultry house litter from commercial poultry farms. Unexpectedly, the major reservoir for Class 1 integrons in poultry litter is not their previously identified hosts, Gram-negative Enterobacteriaceae such as Escherichia coli . Rather, integrons and associated resistance genes abound in several genera of Gram-positive bacteria that constitute >85% of the litter community compared with Enterobacteriaceae that comprise <2% of this ecosystem. This finding warrants reexamination of our assumptions about the persistence and spread of antibiotic resistance genes.

Antimicrobial Agents and Chemotherapy, 2008

Antimicrobial Agents and Chemotherapy, 2005

The Indian Journal of Pediatrics, 2002

Objective : Group A I~ hemolytic streptococcus (GAS) sore throat primarily occurs among children ... more Objective : Group A I~ hemolytic streptococcus (GAS) sore throat primarily occurs among children in 5-15 years age group, and if not treated appropriately causes rheumatic fever/.rheumatic heart disease (RF/RHD). Present study was aimed at validation of a clinical scoring system for diagnosis of GAS. Methods : Five hundred and thirty six children in 5-15 years age group were enrolled by systematic random selection of.households from a peri-urban slum of Chandigarh. They were visited fortnightly at their home for one year to record signs and symptoms of cough and cold. Throat swabs were collected in 918 episodes, of which 123 (13.4%) were GAS culture positive. Result: Significant association of GAS was found with pain in the throat, enlarged tonsils, pharyngeal erythema and tender cervical lymphadenopahty. According to the percentage positivity of GAS culture, weighted scores were assigned to age of the child, season of occurrence, fever, size of tonsil, pharyngeal erythema and exudate, lymphadenopathy and pain in throat. Combinations of various symptoms and signs gave sensitivity of 86-89% and specificity of 83-89% whereas clinical score of 15 or more had 91% sensitivity and 98% specificity for diagnosis of GAS pharyngitis. Conclusion : As the level of clinical acumen and prevalence of GAS may differ in different primary care settings of the country, the proposed scoring system should be validated and adapted to suit local conditions before establishing it in the primary prophylaxis strategy to prevention of RF/RHD.