Indian Journal of Pathology and Microbiology

: 2009  |  Volume : 52  |  Issue : 1  |  Page : 52--55

Trends in antimicrobial resistance of fecal shigella and Salmonella isolates in Tehran, Iran

Mohammad Taghi Hghi Ashtiani1, Maryam Monajemzadeh1, Leila Kashi2,  
1 Associate Professor of Clinical and Anatomical Pathology, Tehran University of Medical Sciences, Tehran, Iran
2 Microbiologist,Tehran University of Medical Sciences, Tehran, Iran

Correspondence Address:
Maryam Monajemzadeh
Keshavarz Boulevard. Children Medical Center Hospital, Pathology Department Tehran


Context: The resistance of bacteria to commonly prescribed antibiotics is increasing both in developing as well as developed countries. Resistance has emerged even to newer, more potent antimicrobial agents. The present study was therefore undertaken to report resistance rates to antimicrobial agents in 2487 stool culture isolates in a tertiary care hospital between 1996 and 2000 and 2001 and 2005. Materials and Methods: During 1996 to 2005, 31776 fecal samples were collected from all patients having diarrhea aged >1 month to 14 years old. Microbiology records were reviewed and information on each isolate regarding its antimicrobial susceptibility profile was collected and recorded. Statistical Analysis Used: The statistical analysis was performed using SPSS, Version 11.5 software. Results: Of the positive fecal cultures, 1329 (53.43%) of the isolates were Shigella spp and 700 (28.14%) of the isolates were Salmonella spp. Resistance to antimicrobial agents increased among most of the pathogens between 2001 and 2005. An increase in the rate of resistance was observed in Shigella spp for kanamycin (from 11 to 37%) and ceftazidime (from 1 to 9.9%) and among Salmonella spp. for nalidixic acid (from 9.2 to 42.3%) and ceftazidime (from 3 to 23.4%). Conclusions: Routine surveillance of antimicrobial susceptibilities to all classes of clinically used agents is necessary to detect resistance trends in different parts of world, detecting the emergence of new resistance mechanisms that guide infection control measures and public health guidelines; such trends may help in identifying outbreaks of resistant organisms. Such a check seems to be the best way to find appropriate antibiotic regimens

How to cite this article:
Ashtiani MT, Monajemzadeh M, Kashi L. Trends in antimicrobial resistance of fecal shigella and Salmonella isolates in Tehran, Iran.Indian J Pathol Microbiol 2009;52:52-55

How to cite this URL:
Ashtiani MT, Monajemzadeh M, Kashi L. Trends in antimicrobial resistance of fecal shigella and Salmonella isolates in Tehran, Iran. Indian J Pathol Microbiol [serial online] 2009 [cited 2021 Jan 21 ];52:52-55
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Infectious diarrhea still causes significant morbidity in the pediatric age group. This problem is especially acute in developing countries, where 0. 25% of all deaths in children aged less than 5 years old are associated with an acute infectious diarrhea. [1]

Fluid and electrolyte replacement by oral hydration or intravenous fluid therapy is the treatment of choice for this disease; however, antibacterial agents are also indicated. [2]

Resistance to many antibacterial agents is increasing in many species of Gram-negative, Gram-positive and anaerobic bacteria. [3] Multi-drug resistant strains have arisen in a multitude of bacterial species including most species of enterobacteriaceae. This is important because of their potential for widespread dissemination, acquisition of additional resistance elements and complications in the therapeutic management of patients. Access to current antimicrobial susceptibility data is of importance to clinicians and is of particular significance to physicians treating hospitalized patients.[4]

Knowledge about susceptibility patterns of bacteria in different geographic areas is necessary to control bacterial resistance.[5]

Moreover, updated bacterial susceptibility data are particularly crucial to physicians and infection control practitioners in countries such as Iran where over-the-counter antimicrobial consumption and abuse of prescribed antibiotics are widespread.

The present study was undertaken to determine the antimicrobial susceptibility profiles of stool culture isolates from patients referred to the Children's Medical Center (CMC) from 1996 to 2000 and to compare them with the susceptibility pattern between 2001 and 2005.

 Materials and Methods

Between 1996 and 2005, 31776 fecal samples were collected from all patients having diarrhea aged >1 month to 14 years old attending CMC in clean, open-mouth disposable containers. All the samples were cultured within 30 minutes of collection and analyzed according to standard methods in the Laboratory of Clinical Microbiology at CMC.

In addition to being a referral tertiary care center, CMC is one of the major teaching hospitals of the Tehran University of Medical Sciences. It admits patients from all regions of Iran, representing a wide spectrum of socioeconomic levels.

Microbiology records were reviewed and information on each isolate regarding its antimicrobial susceptibility profile was collected and recorded. The study protocol was reviewed and approved by the Institutional Review Board of Tehran University of Medical Sciences.

The samples were primarily cultured on Eosin Methylene Blue and Salmonella - Shigella agar and also Enrichment broths . Further isolation and identification of microorganisms were carried out by standard procedures. [6] Susceptibility was determined by the disk diffusion method, following National committee for clinical laboratory standards (CLSI) recommendations. [7],[8] Mueller-Hinton agar (Oxoid, Basingstoke, United Kingdom) was used for all strains. The disks were purchased from Patan-Teb Company, Iran. Incubation was carried out at 35 to 37°C for 18 h in an aerobic atmosphere for all strains. The antibiotic discs included ampicillin, ceftizoxime, chloramphenicol, kanamycin, gentamicin, tobramycin, trimethoprim-sulfamethoxazole, nalidixic acid, cephalothin, amikacin, ceftriaxone and ciprofloxacin. Zones of inhibition were recorded in millimeters and the results of the susceptibility testing were classified into two categories: "susceptible and resistant". Quality control organisms were utilized routinely in the CMC microbiology laboratory to ensure accurate performance of the susceptibility tests.

The statistical analysis was performed using SPSS, Version 11.5 (SPSS Inc., Chicago, IL, USA). Values were tested for statistical significance using chi-square test. A p-value of 0.05 or less was considered significant.


Of the fecal specimens examined, 2487 cases yielded positive cultures among which 1329 (53.43%) yielded Shigella spp. and 700 (28.14%) yielded Salmonella spp. The remaining were E. coli, Pseudomonas spp. , Aeromonas spp. , Enterobacter spp. , Morganella morgani a nd Yersinia enterocolitica .

Comparison of the antimicrobial resistance of Shigella and Salmonella isolates between 1996 and 2000 (n = 916) and 2001 and 2005 (n = 1571) is depicted in [Table 1]. Resistance to antimicrobial agents increased during 2001-2005. [Figure 1] indicates the resistance rates to kanamycin, nalidixic acid, ampicillin, amikacin, ceftazidime and tobramycin among Shigella spp. during the 10-year period. As shown in [Figure 1], the resistance to kanamycin increased gradually from 11% to 37% (p-value=0.0001) and resistance to ceftazidime increased from 1-9.9% reaching statistical significance (p-value=0.0001). [Figure 2] specifies the resistance rates to kanamycin, nalidixic acid, ceftizoxime, ceftriaxone and ceftazidime among Salmonella spp. between 1996 and 2005. As shown in this figure, the resistance to kanamycin and nalidixic acid increased gradually from 15.2 to 44.7%, 9.2 to 42.3% and 2.4 to 31.9%, respectively ( P -value =0.0001). No increased resistance to ciprofloxacin was found among Shigella and Salmonella isolates.

Rates of susceptibility to chloramphenicol, gentamicin and trimethoprim-sulfamethoxazole were steady for Shigella isolates from 1996 to 2005. There were fairly stable rates of susceptibility of Salmonella spp. to cephalothin, gentamicin and trimethoprim-sulfamethoxazole throughout the 10-year study period.

There was no discrepancy in the susceptibility of isolates against both cefotaxime and ceftizoxime; the pattern of susceptibility was very similar ( P [2],[9] especially in developing countries. Most bacterial agents that cause diarrhea in children are Shigella , Salmonella , Escherichia coli, Campylobacter, Yersinia enterocolitica, Clostridium perfringens, Clostridium difficile and Aeromonas . [10],[11],[12] According to our study, the most common bacterial agents of infectious diarrhea in this region are as follows: Shigella spp. 53.43% and Salmonella spp. 28.14%.

Reports from other developing countries showed very high rates of resistance to Shigella spp. from antibiotics that are commonly available in their countries. [13],[14],[15],[16],[17],[18],[19],[20] The present study demonstrated fairly high rates of resistance to Salmonella and Shigella spp. from various antibiotics during the 10-year study period. Cephalothin, ceftizoxime and ceftriaxone resistance among Shigella spp. has steadily increased during the period between 1996 and 2005. A study by Naik, et al. [21] showed high rates of resistance against ampicillin, chloramphenicol and trimethoprim-sulfamethoxazole over a period of 3 years; 6% of the S. flexneri isolates were found to be resistant to nalidixic acid. [21]

Since 1996 when data were first collected, ampicillin, amikacin and chloramphenicol resistance among Salmonella spp. has increased from 60.3% to 72.2%, 2.3% to 9.2% and 17.2% to 27.9% in 2005, respectively. In contrast, there has been no significant change in cephalothin, gentamicin, trimethoprim-sulphamethoxazole and ceftazidime resistance. This may be because of a lower rate of administration in comparison with the first group of antibiotics mentioned above.

Prats, et al . [19] who were studying Salmonella e, found an increase in the rates of resistance to ampicillin (from 8 to 44%), tetracycline (from 1 to 42%), chloramphenicol (from 1.7 to 26%), trimethoprim-sulfamethoxazole (from 0.5 to 11%) and nalidixic acid (from 0.1 to 11%) throughout their 7-year study. [19] Resistance rates among non typhoidal Salmonella spp. has been reported by other studies in India and Spain. [22],[23],[24]

Quinolones are efficient against causes of most bacterial gastroenteritis and are often recommended as empirical therapy but they are not approved for children because of the potential risk of damage to growing cartilage. [25] Resistance to fluoroquinolones has been rarely reported and almost all Shigella isolates are susceptible to them. [26] Our results showed no increased resistance to ciprofloxacin among Shigella and Salmonella isolates.

Among Shigella spp., resistance to nalidixic acid was found to have increased gradually from 4.3% to 10.2%. According to Taneja, et al . [2] resistance to nalidixic acid has increased from 7.4% between 1994 and 1998 to 63.3% between 2000 and 2002. This may be due to the widespread use of this drug as the first choice for empiric treatment of infectious diarrhea in pediatrics. [27]

A significant limitation of the present study was that it was not possible to determine antibiotic resistance rates among isolates from adolescents and adults. The observed rates may be over-estimates as antibiotic use generally tends to decrease with age. Other limitations were the unavailability of information about other variables (e.g., previous antibiotic use, previous hospitalization, other underlying disease) that are associated with the isolation of antibiotic-resistant bacteria. Age is associated with antibiotic resistance in that younger children are more likely than older children to yield resistant strains. [13]

Routine surveillance of antimicrobial susceptibility to all classes of clinically used agents is necessary to detect resistance trends in different parts of world. This would help in detecting the emergence of new resistance mechanisms, working modalities for infection control and formulating public health guidelines. Such an exercise also helps in identifying outbreaks of resistant organisms and the data generated can help in advocating appropriate antibiotic regimens.


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