Third-generation cephalosporin resistance in Shigella sonnei, Argentina.

Shigella sonnei resistant to cefotaxime (but not to ceftazidime) was isolated for the first time in stool samples from a pediatric patient with vomiting and bloody diarrhea in northern Argentina. Microbiologic and biochemical tests confirmed the presence of an extended spectrum beta-lactamase displaying an apparent isoelectric point value of 8.2.

Shigellosis is an important public health problem, especially in developing countries. Antibiotic treatment of bacterial dysentery, aimed at resolving diarrhea or reducing its duration, is especially indicated whenever malnutrition is involved. First-line drugs include ampicillin and trimethoprimsulfamethoxazole (TMP-SMX); however, multidrug-resistant (i.e., resistant to ampicillin, TMP-SMX, and chloramphenicol) Shigella isolates are ubiquitous (1)(2)(3). When epidemiologic data indicate a rise in resistance, fluoroquinolones may be used in adults and oral third-generation cephalosporins in children. Except for a single strain in Calcutta (4), Shigella have not obtained any extended-spectrum beta-lactamase (ESBL)(4), even though multidrug-resistance plasmids were described in this genus as early as the late 1950s (5,6).
A new family of ESBLs, displaying greater affinity for cefotaxime than ceftazidime (CTX-M), has appeared in various countries. In Argentina, CTX-M-2 is the most prevalent ESBL (7) and has been reported in several enterobacteria (8)(9)(10). Although very common in nosocomial pathogens, it has also been recovered from other enteric microorganisms (11). We describe our analysis of the first Shigella sonnei isolate resistant to cefotaxime (CTX) but not to ceftazidime.

The Study
The isolate was from stool samples from a 6-month-old girl, seen in the outpatient clinic of Hospital SAMIC Oberá in northern Argentina. The child had vomiting and laboratoryconfirmed bloody diarrhea approximately 20 days after a 1week hospital stay for diarrhea and primary malnutrition. During the hospital stay, she received gentamicin for 1 week, but no bacterial enteropathogen was isolated.
The child lived in Oberá, a subtropical area in Misiones that has no running water. In this region, Shigella spp. is three times more prevalent than Salmonella spp. and other enteropathogens as the cause of pediatric diarrhea, and S. sonnei resistance to ampicillin and TMP-SMX is approximately 43% and 74%, respectively.
S. sonnei was isolated on eosin-methylene blue agar plates. The isolate did not produce gas and was hydrogen sulfide-negative and nonmotile. In triple sugar iron agar, it was negative for citrate, phenylalanine deaminase, and indol production. It was methyl-red positive, Voges-Proskauer negative, ornithine decarboxylase positive, arginine dihydrolase negative, lysine decarboxylase negative, and urease (Christensen) negative. (All media were from Britania, Argentina.) Confirmatory serotyping was carried out with antisera from the Instituto Nacional de Microbiología Dr. Carlos Malbran.
Resistance to gentamicin, amikacin, and TMP-SMX was detected by agar diffusion (13); quality controls also included E. faecalis ATCC 29212.

Conclusions
The presence of an ESBL was confirmed by microbiologic and biochemical tests using different third-generation cephalosporins as substrates for the enzymes present in bacterial sonicates and an iodometric detection system (9). A microbiologic confirmation test for ESBLs was performed according to recommendations of the National Committee for Clinical Laboratory Standards for E. coli and Klebsiella spp. (13). After isoelectric focusing (9), crude bacterial extracts rendered two bands that hydrolyzed 500 µg/mL ampicillin Shigella sonnei resistant to cefotaxime (but not to ceftazidime) was isolated for the first time in stool samples from a pediatric patient with vomiting and bloody diarrhea in northern Argentina. Microbiologic and biochemical tests confirmed the presence of an extended spectrum beta-lactamase displaying an apparent isoelectric point value of 8.2.
No resistant Shigella was isolated from the other 124 pediatric patients admitted that month for nonsurgical (85 patients) or surgical reasons (29) nor was a nosocomial outbreak caused by a third-generation cephalosporinresistant enterobacteria detected. The patient recovered. The isolated microorganism was not the likely cause for the patient's first hospitalization as it could not be isolated at that time. Likely alternatives for its acquisition are 1) intestinal selection or acquisition of a resistant enterobacterium and in vivo transference to a freshly acquired Shigella or 2) direct acquisition of the resistant strain from contaminated water (not sustained, as no other resistant Shigella was obtained from the same community in the following year).
These findings, which may portend the spread of serious resistance in Shigella throughout Argentina and beyond, suggest the need for susceptibility testing of all Shigella spp. whenever financially feasible.