|Year : 2013 | Volume
| Issue : 3 | Page : 287-289
Antimicrobial resistance pattern and in vivo activity of azithromycin in Salmonella isolates
A Garg1, S Verma1, A Kanga1, D Singh1, B Singh2
1 Department of Microbiology, Indira Gandhi Medical College and Hospital, Shimla, Himachal Pradesh, India
2 Department of Community Medicine, Indira Gandhi Medical College and Hospital, Shimla, Himachal Pradesh, India
|Date of Submission||15-Feb-2013|
|Date of Acceptance||13-Apr-2013|
|Date of Web Publication||25-Jul-2013|
Department of Microbiology, Indira Gandhi Medical College and Hospital, Shimla, Himachal Pradesh
Source of Support: None, Conflict of Interest: None
We evaluated antimicrobial susceptibility pattern of 42 Salmonella isolates from February 2012 through January 2013. We also determined the minimum inhibitory concentrations (MICs) of azithromycin against Salmonella isolates and compared them with corresponding disc diffusion sizes. Entire 42 isolates were sensitive to chloramphenicol, 41 (97.6%) were sensitive to cotrimoxazole and amoxicillin each. MICs for azithromycin ranged from 2 μg/ml to 24 μg/ml, corresponding zone diameters ranged from 15 mm to 33 mm and the two were significantly correlated (P = 0.001). Our results indicate that whereas, azithromycin is a potential therapeutic option, the sensitivity to the first line drugs and absence of multidrug resistance reinforce the concept of antimicrobial recycling.
Keywords: Azithromycin, minimum inhibitory concentrations, resistance, Salmonella, susceptibility
|How to cite this article:|
Garg A, Verma S, Kanga A, Singh D, Singh B. Antimicrobial resistance pattern and in vivo activity of azithromycin in Salmonella isolates. Indian J Med Microbiol 2013;31:287-9
|How to cite this URL:|
Garg A, Verma S, Kanga A, Singh D, Singh B. Antimicrobial resistance pattern and in vivo activity of azithromycin in Salmonella isolates. Indian J Med Microbiol [serial online] 2013 [cited 2020 Aug 8];31:287-9. Available from: http://www.ijmm.org/text.asp?2013/31/3/287/115641
| ~ Introduction|| |
Enteric fever caused by Salmonella enterica serovar Typhi and Paratyphi A, B and C is one of the most common cause of systemic infections in India.  The World Health Organization has estimated that annually typhoid fever accounts for 21.7 million cases and paratyphoid accounts for 5.4 million cases globally.  Changing antimicrobial resistance pattern has become an issue at stake in treatment of enteric fever as delay in the initiation of appropriate antimicrobial treatment increases morbidity and mortality. Therapeutic options for ciprofloxacin resistant enteric fever narrowed down to newer quinolones, extended spectrum cephalosporins, azithromycin, tigecycline and carbapenems.  The efficacy of newer quinolones in ciprofloxacin resistant cases is doubtful. Alternative regimes with extended spectrum have a disadvantage of high costs or intravenous administration and prolonged defervescence. Azithromycin is clinically thought to be efficacious in resistant cases, but efficacy is not established in the laboratories due to lack of interpretive guidelines for assessing Salmonella species susceptibility towards azithromycin . Use of azithromycin may become crucial in settings of ciprofloxacin resistant isolates of Salmonella speciesWe conducted this prospective study to evaluate the current antimicrobial susceptibility/resistance pattern and compared the minimum inhibitory concentration (MIC) of azithromycin with disc diffusion zone diameters in Salmonella isolates.
| ~ Materials and Methods|| |
We studied all the strains of Salmonella isolate from blood samples of fever patients over a period of 1 year from February 2012 through January 2013. Blood samples collected in brain heart infusion broth were incubated aerobically at 37˚C for 18-24 h. Two subcultures were done on blood agar and MacConkey agar after 24 h and 72 h. Identification of the isolates was done by biochemical tests and specific antisera using standard methods (antisera-Central Research Institute, Kasauli, India).  All isolates were subjected to susceptibility testing against chloramphenicol (30 μg), nalidixic acid (30 μg), ampicillin (10 μg), cotrimoxazole (1.25/23.75 μg), ciprofloxacin (5 μg), ceftriaxone (30 μg) and azithromycin (15 μg) by Kirby-Bauer's disc diffusion technique.  The MIC of azithromycin and ciprofloxacin was determined by Epsilometer test (Himedia). We analysed data using SPSS version 16.0 for windows. We calculated Spearman's rank correlation coefficient (r s) and regression coefficient between disc diffusion and MIC for azithromycin taking MIC as dependent variable and zone diameter by disc diffusion as independent variable. We calculated two tailed P values for the correlation and regression coefficients. A P value of 0.05 or below was considered statistically significant.
| ~ Results|| |
A total of 42 isolates were obtained of which 25 were Salmonella enterica ser Paratyphi A (59.5%) and 17 (40.5%) were Salmonella enterica ser Typhi. All isolates were susceptible to chloramphenicol, 41 (97.6%) were susceptible to cotrimoxazole and amoxicillin each. A single serovar of Salmonella Typhi was resistant to cotrimoxazole and another showed intermediate susceptibility to amoxicillin. None of the isolates exhibited multidrug resistance [Table 1]. 40 (95.2%) isolates were nalidixic acid resistant and the remaining two isolates one each of Salmonella Paratyphi A and Salmonella Typhi were susceptible to nalidixic acid (4.8%). MIC for azithromycin ranged from 2 μg/ml to 24 μg/ml and corresponding zone Standardized regression coefficient (Beta) for MIC for given value of zone diameter was − 0.642 (P ≤ 0.001). MIC values for azithromycin were obtained as 2-4 μg/ml for 19/42 (45.23%) isolates, 6-8 μg/ml for 14/42 (33.33%) isolates and 3/42 (7.14%) isolates each corresponding to MIC values of < 2 μg/ml, 12-16 μg/ml and ≥ 16 μg/ml. The zone diameters for azithromycin disc were detected to be ≥ 15 mm for 39/42 (92.86%) and only 3/42 (7.14%) isolates had zone < 15 mm [Table 2].diameters varied from 15 mm to 33 mm [Figure 1]. MIC and zone diameters for azithromycin had significant negative correlation (r s = −0.549; P ≤ 0.001).
|Figure 1: Correlation between zone diameter and minimum inhibitory concentrations for azithromycin (spearman's rank correlation coefficient = −0.549; P ≤ 0.001)|
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|Table 1: Resistance pattern*of Salmonella isolates to different antimicrobials tested (n=42; S. Typhi=17; S. Paratyphi A=25)|
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|Table 2: Azithromycin MIC values and zone diameters in Salmonella isolates (n=42)|
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| ~ Discussion|| |
Susceptibility tests serve as a guide to clinicians for selecting most appropriate therapeutic agent. In our study, all Salmonella isolates were sensitive to chloramphenicol and none exhibited multidrug-resistant (MDR). Most isolates were susceptible to amoxicillin and cotrimoxazole. These findings contrast with previous study conducted in our set up where MDR was observed in 23.2% isolates.  Our findings corroborate with studies conducted in Pondicherry where 66.0% of the isolates were found to be susceptible to first line antimicrobials.  In the current study 9.5% isolates were ciprofloxacin resistant as per zone diameter assessment whereas 14.3% were resistant by MIC. There is no statistical difference between the results (P = 0.736) by two methods. Increase in MIC not detected by disc diffusion tests is documented to result in delayed response and serious complications. 
The in vitro susceptibility of azithromycin from the present study is concluded as MIC ≤16 μg/ml and the zone diameter by disc diffusion to be ≥15 mm. These MIC values obtained are consistent with various studies conducted in India. , There is no evidence that MIC's are more relevant than susceptibility category results but MIC's have the power to predict efficacy in vivo.  In our study, Salmonella Paratyphi A displayed a tendency towards higher MIC than Salmonella Typhi and a similar pattern was observed in a study performed in Pakistan.  The MIC in our set up is higher than in the western world (4-8 μg/ml), which corroborates with other studies.  Previous studies have reported zone diameter as ≥13 mm that is lower than zone size determined in our study.  The British Society of Antimicrobials Chemotherapy (BSAC) suggests the MIC values of ≤16 μg/ml as sensitive.  In accordance to BSAC guidelines we have three (7.1%) resistant isolates with MIC = 24 μg/ml. This correlates with resistance being detected by zone diameters also <15 mm. The previous Indian literature reported MIC 8-16 μg/ml as sensitive, but when compared with our study it is difficult to comment the susceptibility of 64.3% isolates as these had MIC lesser than 8 μg/ml and the zone diameters were ≥15 mm. This further emphasizes the need for clear-cut interpretative guidelines. Establishment of performance standards for disk susceptibility test guidelines for interpreting azithromycin susceptibility testing against Salmonella enterica would facilitate laboratory reporting of this antimicrobial with confidence. As our inventory of drugs active against enteric fever is rapidly shrinking it necessitates a change towards evidence based treatment. We recommend prescribing first line drugs if isolates are susceptible reinforcing the concept of antimicrobial recycling.
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[Table 1], [Table 2]