Salmonella and shigella species

Eastern Mediterranean Health Journal

Shigella and Salmonella serogroups and their antibiotic susceptibility patterns in Ethiopia

المجموعات المصلية من الشيغيلات والسالمونيلات ونماذج حساسيتها للمضادات الحيوية في إثيوبيا

الخلاصـة: استفرد الباحث في هذه الدراسة المجموعات المصلية من أنواع الشيغيلات والسالمونيلات من مزارع البراز ودرس أنماط حساسيتها للمضادات الحيوية باستخدام الإجراءات المختبرية المعيارية. ومن بين 76 مستفرَدة من أنواع الشيغيلات كانت المجموعة المصلية "بي" (الشيغيلة الفلكسنرية) أكثر الأنواع السائدة (54.0%). ومن بين 37 مستفرَدة من ذراري السالمونيلات كانت المجموعة المصلية "بي" هي الأكثر مصادفة (81.1%). وقد أظهرت مخططات الحساسية للمضادات الحيوية لأنواع الشيغيلات والسالمونيلات مقاومةً مطلقة (100%) للإريثروميسين، ومقاومة مرتفعة تزيد معدلاتها على 75% للأمبيسيلين والسيفالوتين والكلورامفينيكول والتـتـراسكلين. وكانت أنواع السلمونيلات مرتفعة المقاومة للجنتاميسين والسلفوناميد والتريميثوبريم – سلفاميتوكسـازول. وكانــت الشيغـيـلات حساسة للجنتاميسيـن (100%) ولحمض الناليديكسيك (97.3%)، في حين كانت الشيغيلات والسالمونيلات حسَّاسة بنسبة (100%) للنورفلوكساسين.

ABSTRACT In this study, the serogroup and susceptibility patterns of Shigella and Salmonella spp. isolated from stool cultures were determined using standard laboratory procedures. Among the 76 Shigella isolates serogroup B (Sh. flexeneri) was the most prevalent species (54.0%) and among the 37 Salmonella strains serogroup B was also the most prevalent (81.1%). Antibiograms of Shigella and Salmonella spp. showed 100% resistance to erythromycin and high resistance rates (≥ 75%) to ampicillin, cephalothin, chloramphenicol and tetracycline. Salmonella spp. also had high resistance to gentamicin, sulphonamide, and trimethoprim–sulfamethoxazole. Shigella were susceptible to gentamicin (100%) and nalidixic acid (97.3%) and Shigella and Salmonella were 100.0% susceptible to norfloxacin.

Les sérogroupes de Shigella et de Salmonella et leur profil de sensibilité aux antibiotiques en Éthiopie

RÉSUMÉ: Dans cette étude, le sérogroupe et le profil de sensibilité de bactéries Shigella et de Salmonella spp. isolées à partir de coprocultures ont été déterminés grâce à des procédures de laboratoire normalisées. Parmi les 76 isolats de Shigella, le sérogroupe B (Sh. flexeneri) était l’espèce la plus fréquemment retrouvée (54,0 %) et parmi les 37 souches de Salmonella, le sérogroupe B était également le plus représenté (81,1 %). Les antibiogrammes réalisés sur les espèces Shigella et Salmonella ont montré une résistance de 100 % à l’érythromycine et des taux de résistance élevés (≥ 75 %) à l’ampicilline, à la céfalotine, au chloramphénicol et à la tétracycline. Salmonella spp. présentait également une résistance élevée à la gentamicine, au sulfamide et au triméthoprime-sulfaméthoxazole. Shigella était sensible à la gentamicine (100 %) et à l’acide nalidixique (97,3 %) et Shigella et Salmonella étaient sensibles à 100 % à la norfloxacine.

1 Department of Medical Microbiology, Immunology and Parasitology, Faculty of Medicine, University of Addis Ababa, Addis Ababa, Ethiopia (Correspondence to D. Asrat: This e-mail address is being protected from spambots. You need JavaScript enabled to view it ). Received: 08/01/06; accepted: 16/05/06
EMHJ, 2008, 14(4):760-767

Gastroenteritis-causing pathogens are the second leading cause of morbidity and mortality worldwide; it is mainly children under the age of 5 years who are at risk. The organisms responsible are rotaviruses, Norwalk-like viruses, enterotoxigenic Escherichia coli (ETEC), Campylobacter jejuni and Clostridium difficile, Shigella spp., Salmonella spp., Cryptosporidium spp. and Giardia lamblia. These organisms are readily transmitted via food, water, environmental contacts, pets and from person to person, with morbidity rates in developing countries 3-to-6-fold higher than in developed countries [1].

Antimicrobial resistance has complica-ted the selection of antibiotics for the treatment of enteric bacterial pathogens, particularly to commonly used antimicrob-ial agents such as ampicillin, tetracycline and trimethoprim–sulfamethoxazole [2].

In Ethiopia there is a great need to establish the identity and antibiotic suscep-tibility patterns of different bacterial agents which cause enteric infections in order to introduce effective treatment for diarrhoeal illness. This paper reports the results of the serogroups and antimicrobial susceptibility patterns of 76 Shigella and 37 Salmonella strains isolated from the stool cultures of patients and controls, with and without diarrhoea illnesses respectively, in Addis Ababa, Ethiopia.

The source of Shigella and Salmonella strains were stool specimens obtained from patients with diarrhoeal disease and cont-rols without symptoms of diarrhoeal disease who were diagnosed with other illnesses. The nature of the diarrhoeal stool specimens was watery (82.4%), bloody (6.8%), mucoid (8.4%) and mixed (2.4%). From February 1992 to January 1993, a total of 76 Shigella and 37 Salmonella strains were isolated from Tikur Anbassa and Ethio-Swedish Children’s Hospital, Addis Ababa, Ethiopia. All the isolated strains were kept frozen at –20 ºC in 15% tryptone soya broth (Oxoid Ltd., Basingstoke, Hampshire, England) containing 15% (v/v) glycerol.

Frozen Shigella and Salmonella strains were subcultured on MacConkey agar no.2 (Oxoid) and incubated at 37 ºC for 24 hours. These bacteria were identified by their characteristic appearance on the media and further confirmed by the pattern of biochemical reactions using a standard bacterial identification system (API 20E, bioMérieux, Marcy-l’Etoile, France). From a pure culture serogrouping and antimicrobial susceptibility testing were done.

Shigella strains were serogrouped by slide agglutination tests using A1, A2, A3, B, C1, C2, C3 and D antisera (National Bacteriological Laboratory, Stockholm, Sweden). For Salmonella strains serogrouping was done by slide aggluti-nation tests using poly O and groups A, B, C, D, E antisera (NBL, Stockholm, Sweden). These strains were further tested against poly H antisera. Those strains identified biochemically as Salmonella typhi were tested against Vi antisera.

All Shigella and Salmonella strains were tested for their susceptibility to different antibiotics using the agar diffusion method according the methodology described by the National Committee for Clinical Laboratory Standards [3]. A McFarland 0.5 standard suspension of the bacteria in 5 mL of phosphate-buffered saline (Oxoid) was then prepared and swabbed over the entire surface of Petragnani culture medium (PDM Antibiotic Sensitivity Medium II, AB Biodisk, Solna, Sweden) with a sterile cotton swab. The inoculated plates were left at room temperature to dry for 3–5 minutes. With the aid of an automatic dispenser (Oxoid) a set of 10 antibiotic disks (Oxoid) with the following concentrations were then delivered to the surface of the PDM II plate: ampicillin 10 μg; cephalothin 30 μg; chloramphenicol 30 μg; erythromycin 15 μg; gentamicin 30 μg; nalidixic acid 30μg; norfloxacin 10 μg; sulfonamide 300 μg; tetracycline 30 μg and trimethoprim–sulfamethoxazole (TMP–SXT) 25 μg. The disks were gently pressed onto the medium with sterile forceps to ensure firm contact. Following overnight incubation at 37 ºC, clear zones produced by antimicrobial inhibition of bacterial growth were measured to the nearest millimetre using metal callipers. The zone diameter was interpreted using an interpretive chart defined by the Clinical and Laboratory Standards Institute [4].

A reference strain of E. coli (ATCC 25922) was used as a quality control for culture and susceptibility testing.

The criteria used to select the antimic-robial agents tested were based on availa-bility and frequency of prescriptions for the management of enteric bacterial infections in Ethiopia (personal communication).

The results of serogrouping of the 76 Shigella isolates are presented in Table 1. Serogroup B (Sh. flexneri) was the most commonly isolated species (54.0%), followed by group A (Sh. dysenteriae) (22.4%), group D (Sh. sonnei) (15.8%) and group C (Sh. boydii) (7.8%). Of the serogroup Sh. dysenteriae, 82.4% were serotypes A1 and 17.6% were type A2. Among serogroup Sh. boydii the prevalence of serotypes were C1 (33.3%), C2 (50.0%) and C3 (16.7%). Of the 76 Shigella isolates, 74 were recovered from patients and 2 from controls (1 Sh. dysenteriae and 1 Sh. flexneri).

Among the 37 Salmonella strains, the most commonly isolated serogroup was group B (81.1%), followed by group D (S. typhi) (10.8%) and group C (8.1%) (Table 2). All S. typhi isolates were recovered from patients. Of the 37 Salmonella isolates, 24 were recovered from patients and 13 from controls.

The results of antimicrobial susceptibility patterns of the Shigella and Salmonella isolates are shown in Table 3. Antibiograms of Shigella species showed that most strains were resistant to ampicillin (78.7%), cephalothin (86.7%), chloram-phenicol (74.7%), erythromycin (100.0%), sulfonamide (54.7%), tetracycline (97.3%) and TMP–SXT (45.3%), but susceptible to gentamicin (100%), nalidixic acid (97.3%) and norfloxacin (100.0%). The Salmonella species were resistant to ampicillin (81.2%), cephalothin (86.4%), chloramphenicol (83.7%), erythromycin (100.0%), gentamicin (75.6%), nalidixic acid (37.8%), sulfonamide (81.1%), tetracycline (94.5%) and TMP–SXT (75.7%). All strains were susceptible to norfloxacin (100.0%). Among Salmonella spp. a comparatively low level of resistance (20%–25%) was detected in S. typhi to all antimicrobial agents tested ex-cept for erythromycin. Multidrug resistance (2 or more antibiotics) was noted in 80%–90% of both isolates (data not shown).

In this study, serogroup B (Sh. flexeneri) was the dominant Shigella serogroup, followed by group A (Sh. dysenteriae), group D (Sh. sonnei) and group C (Sh. boydii). These findings are in accordance with previous Ethiopian studies, except that in those studies Sh. boydii was the 3rd most commonly isolated species [5–8]. It is not unusual for one serogroup to replace another in the community from time to time. The comparative frequencies of Shigella serogroups vary with time, hygienic conditions and among different popula-tions. In the early 1900s Sh. dysenteriae type 1 was the most common strain, whereas Sh. flexneri and Sh. sonnei are currently isolated most often, except for certain epidemics in which Sh. dysenteriae has been identified as the causative organism [9,10]. In developed countries, higher frequencies of Sh. sonnei have been reported, but these frequencies are gradually decreasing [11]. Epidemics of dysentery with frequent passage of blood and mucus, high fever, cramps and tense-mus are mainly caused by Sh. dysenteriae type 1 and Sh. flexenri, while Sh. boydii and Sh. sonnei often causing non-watery (often bloody) diarrhoea during non-epidemic episodes [12]. Bennish and Wojtyniak [13] reported most fatal cases of shigellosis occur in developing countries as a result of severe dysentery and in rare cases, bacteraemia, especially that caused by Sh. flexneri.

The susceptibility of Shigella spp. to antibiotics has changed considerably over time. In the 1940s bacillary dysentery was treated successfully with sulfa-drugs and in the 1950s with tetracycline [14]. In the 1970s, resistance to one or more of the antimicrobial agents then in use began to emerge [15], but ampicillin was available and was used successfully to treat shigellosis by that time. When Shigella spp. began to develop resistance to ampicillin, TMP–SXT became the drug of choice [16]. Since 1980, however Shigella spp. have demonstrated a frequent and alarming resistance to TMP–SXT [17]. With the usefulness of these antimicrobials curtailed by the emergence of resistant strains, investigators are challenged to find new alternative drugs.

In this study the Shigella isolates were more susceptible to gentamicin (100%), nalidixic acid (97.3%) and norfloxacin (100%) than to drugs commonly used to treat shigellosis including ampicillin (21.3%) and TMP–SXT (54.7%). In the early 1980s, studies done in Addis Ababa, Ethiopia, indicated that all or most Shigella spp. were susceptible to TMP–SXT (98.0%–100%) and ampicillin (52%–79.0%) [5,6]. Furthermore, O’Brien reported in 1987 that in many areas of the world the susceptibility of Shigella spp. to nalidixic acid and aminoglycosides remains constant, whereas their susceptibility to ampicillin and TMP–SXT has decreased considerably [18]. The present study also revealed that a high level of resistance to cephalothin (86.7%), chloramphenicol (74.7%), erythromycin (100.0%), sulfonamide (54.7%) and tetra-cycline (97.3%). These findings are in ag-reement with the previous data obtained from Ethiopia [7,8] and other developing countries such as Bangladesh [19], and eastern Africa [20]. Similar patterns of anti-microbial susceptibility have been observed in the United States of America [21], Europe and Latin America [22].

Among the Salmonella strains, the most commonly isolated serogroup was group B, followed by group D (S. typhi) and group C. This is an agreement with some previous studies in Ethiopia [6,8], but in contrast to the earlier studies which showed that S. typhi was the dominant species [23,24]. All serogroups of Salmonella isolated in this study are known to cause gastrointestinal infections.

Among all antibiotics tested for Sal-monella spp., the highest resistance was observed with ampicillin (81.2%), cepha-lothin (86.4%), chloramphenicol (83.7%), erythromycin (100.0%), gentamicin (75.6%), sulfonamide (81.1%), tetracyc-line (94.5%) and TMP–SXT (75.7%). These findings are in contrast with those studies done in Ethiopia in the 1980s that showed that most Salmonella spp. were sensitive to the majority of drugs tested (77.8%–98.4%) [6,23–25], but in agreement with those studies done in the 1990s [8,26]. The marked resistance pattern observed in this study also agrees with reports from other parts of the world [22,27,28]. Reports of antimicrobial resistance trends in Salmon-ella isolates by these investigators show that Salmonella has developed resistance to the above antimicrobial agent over the years. It was not possible to include 3rd-generation cephalosporins for susceptibility testing in this study. In Ethiopia, these drugs are not widely used for treatment of salmonellosis/shigellosis. In the near future there is a need to determine the susceptibility pattern for cephalosporins because resistance to these drugs has been increasing, as documented elsewhere [22].

Many factors have contributed to the development of resistance in gastrointes-tinal pathogens, including misuse, overuse, quality and potency of the antimicrobial agents [29]. According to Salyers and Amábile-Cuevas [30], acquiring resistant genes, even from distantly related genera, is what accounts for the development and spread of drug resistance in bacteria. These authors further explained that the ability of resistance genes to adapt rapidly to new hosts so that they are not readily lost even in the absence of antibiotic selection might be the reason why increases in resistance can be so hard to reverse.

In conclusion, periodic evaluation of the susceptibility pattern of Shigella and Salmonella spp. would be particularly useful. In addition, controlled clinical trial studies are needed to verify the demonstra-ted efficacy of alternative drugs in treating shigellosis and salmonellosis. Furthermore, developing a broadly protective vaccine may be a more effective approach to cur-bing morbidity and mortality against these enteric pathogens.

This work was supported partly by the grants available from the Swedish Cooperation with developing countries (SAREC) programme for Bio-Medical Research and Training. I would like to extend my heartfelt thanks to staff members of the Department of Internal Medicine and Pediatrics, Faculty of Medicine, Addis Ababa University for the continued assistance they offered during collection of specimens from patients in their respective departments.

Abstract

Salmonellosis and Shigellosis remain a major public health problem across the globe, particularly in developing countries like Ethiopia, where hand hygiene and food microbiology are still below the required standards. The growing problem of antimicrobial resistance species also continues to pose public health challenges. This study assessed the prevalence and antimicrobial susceptibility pattern of Salmonella and Shigella species among asymptomatic food handlers. A cross-sectional study was conducted among 417 randomly selected asymptomatic food handlers. Data were collected using a structured questionnaire. The stool specimens collected were examined for Salmonella and Shigella species using standard bacteriological methods. Descriptive statistics were used to describe the basic features of the data.

The overall prevalence of Salmonella and Shigella species was 5.04%. Salmonella and Shigella species were 76.2% resistant to both co-trimoxazole and tetracycline, 71.4% to amoxicillin and 66.7% to chloramphenicol. Moreover, 85.7% of Salmonella and Shigella species were multidrug resistant. The findings highlighted the food handlers as potential sources of food borne infections, which demands the establishment of appropriate hygiene and sanitary control measures at the University cafeterias.

Introduction

Salmonella and Shigella species are the most common causes of food and water-borne gastroenteritis in humans, which remains an important health problem worldwide [1, 2]. According to World Health Organization (WHO) estimates, there are about 16 million new cases and 600,000 deaths from typhoid fever each year worldwide [2]. Shigella strains have also continued to play a major role in the etiology of inflammatory diarrhea and dysentery [3].

The emergence of antimicrobial resistant Salmonella and Shigella spp. are other global challenges, especially in developing countries where there is an increased misuse of antimicrobial agents in humans and animals [2, 4]. An example of the global threat of antimicrobial resistant Salmonella spp. have been widely reported in Europe and America [5]. In Ethiopia, high frequency of resistant Salmonella and Shigella spp. have been observed among the following antimicrobial groups: tetracycline (52.5%, 82.4%), co-trimoxazole (37.5%, 76.5%) and ampicillin (60%, 47.1%), respectively [6].

Salmonella and Shigella spp. are commonly transmitted through the feco-oral route and close contact with infected individuals [7]. Asymptomatic food handlers are known to play crucial role in transmitting the infections and continue to pose a threat to public health [4, 8]. Previous reports indicate that food prepared in large quantities by the involvement of a large number of food handlers at higher learning institutions has been often prone to contamination by infected or asymptomatic carriers of infections leading to outbreaks of food borne diseases [9].

In addition, isolation of Shigella and Salmonella spp. in most parts of African laboratories, including Ethiopia remain a challenge, due to inadequate laboratory facilities to allow accurate detection and performance of antimicrobial susceptibility testing [10]. As a result, information relating to Salmonella, Shigella spp. and their antimicrobial susceptibility patterns in Ethiopia are scarce. Thus, this study was aimed to determine the prevalence and antimicrobial susceptibility patterns of Salmonella and Shigella spp. among food handlers working at Haramaya University cafeterias.

Main text

A cross-sectional study was conducted among asymptomatic food handlers working at Haramaya University, Eastern Ethiopia from August 2015 to January 2016. The University is located at a distance of 510 km from Addis Ababa. Currently, the University cafeterias serve meals for 30,000 students and staffs.

The sample size was calculated using a single population proportion formula by taking an estimated 50% proportion (p = 0.5) of Salmonella and Shigella spp., 5% margin of error (d = 0.05) and 95% confidence interval (z = 1.96). The initial sample size was 384, and by considering 10% non-response rate, the final sample size was determined to be 422. To select representative participants, the final sample size was proportionally allocated to each stratum, and food handlers were selected using systematic random sampling technique. Participants who reported to have never used any antimicrobial in the last 2 weeks and during the study period were included in the study. A complete list of food handlers was obtained from the human resource management directorate of Haramaya University.

A pre-tested structured questionnaire was used to collect data relating to sociodemographic characteristics of the study participants. The questionnaire was developed from validated tools [4, 11, 12].

The stool specimens were collected after brief instruction in sterile, leakproof test tubes containing a Cary-Blair transporting media (CM0519B, Oxoid, Ltd, UK), and transported in the temperature controlled cold box to the microbiology laboratory unit of the College of Health and Medical Sciences for bacteriologic analysis.

Culture isolation and identification of Salmonella and Shigella spp. was performed based on the standard procedure [13]. Briefly: 25 g of stool specimen was homogenized in 225 ml of buffered peptone water (CM1049B, Oxoid, Ltd, UK) using blender (Stomacher 400, Seward, England) for 1 min. A volume of 1 ml aliquot was transferred into 10 ml of Selenite F broth (CM0399B, Oxoid, Ltd, UK) and incubated at 37 °C. After overnight incubation, a loopful of culture was taken and streaked on Xylose lysine deoxycholate agar plate (TV5028N, Oxoid, Ltd, UK) and incubated overnight at 37 °C. The culture plates were examined for the presence of Salmonella spp. (Pink–red with a black center colonies) and Shigella spp. (Pink-red colonies) [13, 14]. Culture positives were characterized by standard biochemical tests, including motility (CM0435B, Oxoid, Ltd, UK), indole test (CM0967B, Oxoid, Ltd, UK), and Kligler iron agar reactions (CM0033B, Oxoid, Ltd, UK). The result of each culture was read after incubation for 24–48 h at 37 °C [15, 16]. The morphology of the isolate was also characterized microscopically using the Gram staining technique [15].

The antimicrobial susceptibility test was done using the modified disk diffusion technique on Mueller–Hinton agar (PO5007A, Oxoid, Ltd, UK) in accordance with the Clinical and Laboratory Standards Institute (CLSI) guideline [17]. In brief, about 3–5 colonies of the same type were picked up from culture media and mixed with 5 ml of sterile normal saline and standardized to 0.5 McFarland. A sterile cotton swab was used to distribute the bacterial suspension evenly over the entire surface of the Mueller–Hinton agar plates. Then, antimicrobial disks were applied to the surface of the inoculated plates using an automatic disk dispenser (ST6090, Oxoid, Ltd, UK). Nine antimicrobial disks (Oxoid, Ltd, UK) including ampicillin (10 mg), tetracycline (30 mg), chloramphenicol (30 mg), gentamicin (10 mg), ciprofloxacin (10 mg), co-trimoxazole (1.25/23.75 µg), ceftazidime (30 µg), norfloxacin (10 mg) and ceftriaxone (30 µg) were applied on the culture plates. The zone of inhibition was read after 24 h of incubation at 37 °C [17]. Multidrug resistance (MDR) is defined when isolates are resistant to two or more antimicrobials of different group [18].

The questionnaire was first prepared in English and translated into two local languages (Amharic and Afan Oromo) and then translated back to English by different bilingual experts to check the consistency. The questionnaire was reviewed by Medical Microbiologists, and pre-tested on 5% of the food handlers working in Dire Dawa University cafeterias to check the practicability and the applicability of the questionnaire.

Data collectors and supervisors were trained for 2 days on the objective of the study, interviewing techniques, and data quality management. Regular supervision, spot checking and reviewing the completeness and consistency of questionnaires on a daily basis were made to assure the quality of data.

The quality of each new batch of culture medium and antimicrobial disks was checked by testing E. coli (ATCC ® 25922), S. aureus (ATCC ® 25923), and P. aeruginosa (ATCC ® 27853) reference strains. All testing results obtained from the reference strains were within the established quality control limits of the CLSI guideline [17]. The result of the culture and antimicrobial tests was read by two Medical Microbiologists.

The data were checked for completeness, cleaned and double entered into Epi-Info version 3.5.1 (CDC, Atlanta, GA, USA). The two entries were compared, and discrepancies were resolved by referring to the original document and exported to the Statistical Package for Social Sciences (SPSS) version 20 (Inc, Chicago, IL) for analysis. Results were presented as percentages, mean, and standard deviation.

A total of 417 asymptomatic food handlers were enrolled in this study, making a response rate of 98.8%. Of these, 79.4% were female; with a male to female ratio of 0.26:1. The majority of participants age was between 31 and 40 years (39.3%) with the mean age of 36.1 (Standard deviation ± 8.7 years). Up to 42.4% were literate with primary level education (1–8th grade). The majority of the participants was currently married and served as a food handler for more than 5 years (58.5%) and (66.2%), respectively (Table 1).

The overall prevalence of Salmonella and Shigella spp. was 5.04%. Of these, 3.6% and 1.4% were Salmonella and Shigella spp., respectively. The most frequently isolated Salmonella spp. was S. typhi (2.2%) followed by S. paratyphi (1%).

Salmonella and Shigella spp. were more prevalent among females (76.2%), age more than 40 years (52.4%), had no formal education (38.1%), were married (57.1%) and had been working at the cafeteria for more than 5 years (61.9%) (Table 1).

More than 85% of Salmonella and Shigella isolates were sensitive to both Ceftazidime and Ciprofloxacin, and 81% to both Ceftriaxone and Norfloxacin. A higher rate of resistance (76.2%) was observed to both Co-trimoxazole and Tetracycline, 71.4% to Ampicillin and 66.7% to Chloramphenicol (Table 2).

Salmonella typhi was highly sensitive to ceftazidime (100%), norfloxacin (88.9%), 77.8% to both ceftriaxone and gentamicin; whereas there was 88.9% resistance to chloramphenicol, co-trimoxazole and tetracycline. On the other hand, Shigella species were 100% sensitive to ciprofloxacin and 83.3% sensitive to both ceftriaxone and norfloxacin; while resistance to tetracycline was 83.3% and co-trimoxazole was 66.7% (Table 2).

The overall multidrug resistance Salmonella and Shigella spp. was 85.7%. Of this, 88.9% was Salmonella spp. Among Salmonella spp., the most frequent MDR isolates were S. typhi (50%) (Table 3).

In this study, the overall prevalence of S. typhi was 3.6%. The finding is comparable to a study conducted in Addis Ababa University, Ethiopia (3.4%) [19]; but relatively higher than reports from other parts of Ethiopia, such as Dilla University (0.93%) [8] and Mekelle University (1%) [12]. However, it is lower compared with a study done in Arba Minch University, Southern Ethiopia (6.9%) [20] and Bahir Dar, Ethiopia (80%) [4]. On the other hand, a higher rate of Shigella spp. (1.4%) was isolated in this study. This is much more than an expected for the occurrence of a bacillary dysentery outbreak. However, the prevalence is lower compared to reports elsewhere from Ethiopia such as in Gondar University (2.7%) [11] and Arba Minch University (3%) [20]. The possible explanation for this variation might be due to differences in the sample size (small sample size might overestimate the proportion), geographical variation and socioeconomic conditions.

Salmonella typhi showed a high sensitivity to ceftazidime (100%), norfloxacin (88.9%), ceftriaxone (77.8%) and norfloxacin (66.7%); whereas there was 88.9% resistance to chloramphenicol, tetracycline and co-trimoxazole. This is comparable to studies conducted in other parts of Ethiopia such as in Mekelle University, where S. typhi showed 100% sensitivity to norfloxacin and 75% resistance to ampicillin, tetracycline and chloramphenicol [12] and in a Gonder University in which there was 100% sensitive to norfloxacin and 50% resistance to both tetracycline and co-trimoxazole [11], indicating that antimicrobial resistance of S. typhi is an increasing concern.

In the current study, Shigella spp. were 100% sensitive to ciprofloxacin, 83.3% to both ceftriaxone and norfloxacin; while 83.3% were resistant to tetracycline. This is consistent with reports from Gonder University, Ethiopia, where Shigella species showed high level of sensitivity to ciprofloxacin (100%) and norfloxacin (87.5%), but with 75% resistance to tetracycline [11]. The similarity in the antimicrobial susceptibility pattern among Shigella spp. may be due to the availability and unrestricted use of antimicrobials [18].

This study also showed high MDR (85.7%) Salmonella and Shigella spp., where MDR was higher among the Salmonella spp. (88.9%), and less among the Shigella spp. (11.1%). The MDR Salmonella spp. was low compared to a report from Addis Ababa University, Ethiopia (100%) [22] but higher compared to a study conducted in Gonder University (46.2%) [13]. On the other hand, the MDR Shigella spp. is also low compared to studies conducted in other parts of Ethiopia such as Addis Ababa University (100%) [21] and Arba Minch University (100%) [14]. The cause of variations in the prevalence of MDR is unknown, but might be due to inappropriate empirical antimicrobial treatment, easy availability and indiscriminate use of common antimicrobials.

The study indicates a higher prevalence of Salmonella and Shigella spp. among food handlers who had more than 5 years’ work experience (61.9%) compared to less than or equal to 5 years (38.1%). This is in line with a study done in Arba Minch University, Ethiopia (32.4%) [20]. However, the results are lower compared to a study done in Mekelle University, Ethiopia in which 60% of food handlers who served for less than 5 years were infected [12]. Lack of regular medical checkups, food safety training, inadequate supervision and a low level of literacy of food handlers might contribute to this difference. The prevalence of antimicrobial resistant Salmonella and Shigella spp. in this study is high. Infected or asymptomatic carriers who are handling food on a daily basis can act as sources of infection to consumers via the food chain. A periodic medical check-up program with intensive health education could improve the workers’ health status.

Fingernail content examination, which might support the idea of contamination due to poor food handling practices, was not performed. In spite of this limitation, the methods used to isolate and characterize the antimicrobial susceptibility pattern of Salmonella and Shigella spp. are comprehensive.

Abbreviations

Clinical and Laboratory Standards Institute