Emerging nalidixic acid and ciprofloxacin resistance in non-typhoidal Salmonella isolated from patients having acute diarrhoeal disease.

BACKGROUND: Non-typhoidal Salmonella are one of the key etiological agents of diarrhoeal disease. The appearance of multiple drug resistance along with resistance to quinolones in this bacterium poses a serious therapeutic problem. We determined the prevalence of nalidixic acid and ciprofloxacin resistance in non-typhoidal Salmonella isolated from faecal samples of patients with acute diarrhoeal disease attending the outpatient and inpatient departments of a hospital in Saudi Arabia during the years 1999 to 2002.
METHODS: Non-typhoidal Salmonella were isolated from faecal samples. Antimicrobial susceptibility was tested by the disc diffusion test. MICs to nalidixic acid and ciprofloxacin were determined by the agar dilution method.
RESULTS: During the study period, 524 strains of non-typhoidal Salmonella were isolated. Strains belonging to serogroup C1 were the commonest (41.4%) followed by serogroups B and D (15.6% and 14.5%, respectively). Resistance to ampicillin was observed in 22.9% and to trimethoprim/sulfamethoxazole in 18.5% of the strains. Nalidixic acid resistance was encountered in 9.9% and ciprofloxacin resistance in 2.3% of the strains. Resistance to nalidixic acid significantly increased from 0.1% in 1999 to 5.5% in 2002 (P=0.0007) and ciprofloxacin resistance increased significantly from 0.1% in 1999 to 0.9% in 2002 (P=0.0001). MICs to nalidixic acid and ciprofloxacin were determined among 29 nalidixic acid-resistant strains of non-typhoidal Salmonella isolated during 2002. The MIC was >256 microg/mL to nalidixic acid and 8 to 16 microg/mL to ciprofloxacin.
CONCLUSION: The increasing rates of antimicrobial resistance encountered among non-typhoidal Salmonella necessitate the judicious use of these drugs in humans. Moreover, these findings support the concern that the use of quinolones in animal feed may lead to an increase in resistance and should be restricted.

Foodborne non-typhoidal Salmonella infections have become a major problem in the industrialized and developing countries.1 They are one of the most important causative agents of acute diarrheal disease in children and adults. The World Health Organization has determined that the non-typhoidal Salmonella are emerging as one of the most important etiological agents of infectious diseases in the world.2 The global emergence of multiple drug resistance in non-typhoidal Salmonella is a serious public health concern and is posing a severe problem in treatment of intestinal and extra-intestinal infections due to these organisms.3–6 The rapid international dissemination of these multidrug-resistant strains of non-typhoidal Salmonella among humans suggests that the multidrug resistance is associated with enhanced virulence.7 A study from Denmark has reported 4.8 times higher mortality in patients having infection due to multidrug-resistant Salmonella typhimurium in comparison to infection with sensitive strains.8

Quinolones remain the treatment of choice for non-typhoidal Salmonella infections, after the appearance of multidrug resistance to commonly used antibiotics like ampicillin, trimethoprim/sulfamethoxazole and chloramphenicol. The recent appearance of quinolone resistance in non-typhoidal Salmonella is a matter of great disquiet for treating physicians and microbiologists.9 Resistance to nalidixic acid, which has been associated with reduced efficacy of fluoroquinolones, such as ciprofloxacin, has been reported recently from many countries.10,11 Epidemiological studies have shown that the number of Salmonella isolated with ciprofloxacin resistance are increasing in Europe.12–14 Multidrug-resistant Salmonella typhimurium DT104, also resistant to quinolones, has been responsible for extensive outbreaks worldwide, leading to high mortality.15,16

The present study describes the prevalence of nalidixic acid and ciprofloxacin resistance in strains of non-typhoidal Salmonella isolated from the Al-Hasa region of Saudi Arabia.

Methods

The study was carried out at the 500-bed King Fahad Hospital Al-Hofuf, during the period 1999 to 2002. The non-typhoidal Salmonella were isolated from faecal samples of patients admitted to or attending the out-patient and inpatient departments of the hospital. Faecal samples were cultured directly on xylose lysine deoxycholate agar and inoculated in selenite F broth (Oxoid Ltd, UK), which was subcultured on xylose lysine deoxycholate agar after 24 hours of incubation. Salmonella strains were biochemically identified by the API-20E system (bioMerieux SA, France) and serogrouped using somatic group Salmonella A-G antisera (Murex Biotech Ltd, UK). Antibiotic susceptibility was performed by the disc diffusion technique according to the criteria of National Committee for Clinical Laboratory Standards (NCCLS) 17. Susceptibility tests were done on Mueller Hinton agar (Oxoid Ltd, UK) using the following concentrations (μg/disc) of antibiotics (Becton Dickinson Co, Maryland, USA): ampicillin-25, amoxicillin/clavulanic acid-20/10, cephalothin-30, cefoxitin-30, cefotaxime-30, ceftriaxone-30, chloramphenicol-30, trimethoprim/sulfamethoxazole-1.25/23.75, gentamicin-10, amikacin-30, imipenem-10, aztreonam-30, piperacillin-100, nalidixic acid-30, ciprofloxacin-5. The minimum inhibitory concentration (MIC) for nalidixic acid and ciprofloxacin of the 29 nalidixic acid-resistant strains isolated during 2002 was determined by the agar dilution method.18 Statistical analysis for comparison of data on resistance between years was done by the Chi-square and Fishers exact tests. Statistical significance was set at the 0.05 level.

Results

During the 4-year period, 524 strains of non-typhoidal Salmonella were isolated from the faecal samples of patients having diarrhoeal illness. Salmonella strains belonging to serogroup C1 were the commonest (41.4%) followed by serogroups B and D (15.6% and 14.5%, respectively). Salmonella strains belonging to serogroup G were least frequent (2.7%) (Table 1). Resistance to ampicillin was observed in most strains (22.9%) followed by resistance to trimethoprim/sulfamethoxazole (18.5%). Resistance to chloramphenicol and amoxicillin/clavulanic acid was observed in 8.6% and 5.3% of the strains, respectively Resistance to nalidixic acid was noticed in 9.9% and to ciprofloxacin in 2.3% of the strains. All the isolated strains were sensitive to cephalothin, cefotaxime, ceftriaxone, gentamicin, amikacin, imipenem, aztreonam and piperacillin.

Overall antibiotic resistance among the non-typhoidal Salmonella strains increased from 8.2% in 1999 to 25.3% in 2002. There was a trend for increasing resistance to nalidixic acid and ciprofloxacin during the study period. Resistance to nalidixic acid significantly increased from 0.1% in 1999 to 5.5% in 2002 (P=0.0007) and ciprofloxacin resistance also significantly increased from 0.1% in 1999 to 0.9% in 2002 (P=0.0001) (Table 2). Multiple antibiotic resistance (resistance to two or more of the antibiotics) was observed in 123 (37.2%) of the strains. Resistance was observed most frequently (39.8%) among the strains belonging to serogroup C1, followed by serogroups C2 and B (18.9% and 16.9%, respectively) (Table 3).

Resistance to nalidixic acid was more common among the strains belonging to serogroup C1 than the strains from other serogroups. Of 52 (9.9%) strains resistant to nalidixic acid, 12(2.3%) were also resistant to ciprofloxacin. Isolated resistance to nalidixic acid and ciprofloxacin was observed more frequently. There was a 29 times increase in resistance to nalidixic acid during the study period, from 1 resistant strain in 1999 to 29 strains in. 2002 (Table 4). The MIC of non-typhoidal Salmonella to nalidixic acid and. ciprofloxacin was determined only for the 29 strains isolated during 2002 that were resistant to these antibiotics. The MIC to nalidixic acid was >256 μg/mL in all the strains. The MIC to ciprofloxacin ranged from 8 to 16 μg/mL among all the five strains that were resistant to this antibiotic. Two of the strains resistant to nalidixic acid had reduced susceptibility (MIC 0.20 μg/mL) to ciprofloxacin.

Discussion

Antimicrobial resistance in non-typhoidal Salmonella has increased worldwide as a consequence of excessive use of antimicrobial agents.19 Antimicrobial susceptibility to ampicillin, trimethoprim/sulfamethoxazole and quinolones for the Salmonella isolated from faecal samples should be routinely tested and. reported by clinical laboratories as per the recommendations of NCCLS. Susceptibility to chloramphenicol and cephalosporins is recommended for Salmonella strains isolated from extra-intestinal sources, as the clinical efficacy of other drugs in intestinal infections has not been proven though they may appear susceptible in in-vitro tests. However, susceptibility to other antimicrobial agents in intestinal isolates of Salmonella has epidemiological importance.17 Ampicillin, trimethoprim/sulfamethoxazole and the fluoroquinolones are established as standard first-line therapy for Salmonellosis. The appearance of resistance to these antibiotics in non-typhoidal Salmonella is posing a serious problem in the treatment of infections due to these organisms.9 Resistant Salmonella has a selective advantage in the environment where excessive antibiotics are used and the antibiotic treatment itself is a major risk factor for infection with the resistant bacteria.20,21 The emergence of quinolone resistance in non-ty-