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Food Control 18 (2007) 306311

www.elsevier.com/locate/foodcont

0956-7135/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodcont.2005.10.013

Antimicrobial resistance ofSalmonella spp. and Escherichia coliisolatedfrom table eggs

A. Adesiyun a,, N. OYah a, N. Seepersadsingh a, S. Rodrigo a, V. Lashley b, L. Musai b

a School of Veterinary Medicine, Faculty of Medical Sciences, University of the West Indies, St. Augustine, Trinidad and Tobagob Poultry Surveillance Unit, Ministry of Agriculture, Land and Marine Resources, Port of Spain, Trinidad and Tobago

Received 25 May 2005; received in revised form 18 October 2005; accepted 22 October 2005

Abstract

The antimicrobial sensitivity of Salmonella spp. and Escherichia coliisolated from the shells and contents of table eggs sampled from

sale outlets in Trinidad was determined using the disc diVusion method. The phage types ofS. Enteritidis isolates, the phenotypic charac-

teristics ofE. coliisolates and the presence of O157 strain were also investigated. Of a total of 74 isolates of Salmonella tested, 17 (22.9%)

exhibited resistance to one or more of the seven antimicrobial agents used compared with 104 (88.1%) of 118 E. coliisolates. The diVer-

ence was statistically signiWcant (P < 0.05; X2). For both microorganisms, resistance was relatively high to streptomycin (54.2%) and tetra-

cycline (35.9%) but low to gentamicin (11.5%) and sulphamethoxazole/trimethoprim (9.4%). Only 1 (1.4%) isolate of Salmonella was

multi-resistant while 55 (46.6%) ofE. coliisolates were resistant to three or more antimicrobial agents. The frequency of resistance to anti-

microbial agents amongst both bacteria was not signiWcantly (P > 0.05; X2) aVected by the location of isolation on the egg (shell or con-

tent) or source of eggs (farms, shopping malls or other retailers). Eight (19.5%) of 41 S. Enteritidis isolates tested were resistant compared

to 4 (26.7%) of 15 isolates ofS. Ohio. All S. Enteritidis isolates belonged to phage type 1 (PT1) and all E. coliisolates were non-haemo-

lytic, non-mucoid, sorbitol fermenters and non-O157 strains. It was concluded that the relatively high resistance amongst the bacteriatested could pose therapeutic problems in consumers, particularly in egg-borne salmonellosis or colibacillosis.

2005 Elsevier Ltd. All rights reserved.

Keywords: Antimicrobial resistance; Phage type; Salmonella; E. coli; Table eggs

1. Introduction

Table eggs which constitute several dishes or foods con-

sumed and are considered cheap sources of protein, have

served as vehicles for numerous enteropathogens

(Hangombe, Sharma, Skjerve, & Tuchili, 1999). Salmonellaspp., particularly S. Enteritidis have been most frequently

associated with table eggs (Nygard et al., 2004). Table eggs

contaminated by bacterial pathogens and consumed raw or

improperly cooked have been responsible for many epi-

demics of gastroenteritis worldwide (Nunes et al., 2003;

Nygard et al., 2004). It has been recommended that table

eggs destined for human consumption be properly cooked

or pasteurized prior to consumption (Davies & Breslin,

2003; Indar, Baccus-Taylor, Commissiong, Prabhakar, &

Reid, 1998; Indar-Harrinauth et al., 2001).

Enteric pathogens such as Salmonella, E. coliand other

pathogens isolated from table eggs have been characterizedas to their serotypes, phage types, toxigenicity and antibi-

otic sensitivity (CDC, 1996; Ibeh & Izuagbe, 1986; Indar-

Harrinauth et al., 2001; Radkowski, 2001). Salmonella

Enteritidis phage type 4 (PT4) has been most frequently

associated with egg-borne salmonellosis in many epidemics

worldwide although other phage types have also been

implicated (Berghold, Kornschober, & Weber, 2003; Indar-

Harrinauth et al., 2001; Kowalczyk-Pecka, Wernicki, &

Puchalski, 2003; Mare, Van der Walt, & Dicks, 2000;

Nygard et al., 2004). Pulse-Weld gel electrophoresis (PFGE)

* Corresponding author. Tel.: +1 868 645 4481; fax: +1 868 645 7428.

E-mail address:[email protected] (A. Adesiyun).
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A. Adesiyun et al. / Food Control 18 (2007) 306311 307

has been used to determine the relatedness ofS. Enteritidis

isolates (Cardinale et al., 2005). It has been demonstrated

that E. coliO157 is capable of colonizing the caeca of layers

and subsequently contaminate their eggs (Berry, Doyle, &

Schoeni, 1985).

In Trinidad and Tobago, eggnog containing Salmonella-

contaminated eggs resulted in an outbreak of salmonellosisamongst inmates of a mental hospital, with several mortali-

ties (Hyatali, Quamina, Mohess, & Foster, 1992). Transov-

arial transfer of Samonella in table eggs at poultry farms

was recently demonstrated by Indar et al. (1998). Adesiyun

et al. (2005b) determined the microbial health risk of table

eggs in Trinidad while in another study (Adesiyun et al.,

2005a) the prevalence of macrolides, tetracycline, beta-lac-

tam and sulphonamides in table eggs sampled from sale

outlets across the island emphasizing the risk they pose to

consumers. In view of the fact that table eggs constitute

many foods or drinks consumed locally, some raw or only

partially cooked, the study was conducted to determine the

antimicrobial sensitivity ofSalmonella spp. and E. coliiso-

lates from table eggs in Trinidad, to investigate the phage

types ofS. Enteritidis and Wnally to detect the presence of

O157 strains amongst E. coliisolates.

2. Materials and methods

2.1. Source and collection of table eggs

Table eggs from which the enteric pathogens studied

originated from poultry farms, shopping malls and other

retailers in Trinidad. The sampling protocol and number ofeggs collected from each source were earlier described

(Adesiyun et al., 2005b).

Overall, a total of 46 egg composite samples, made of 25

eggs per composite were sampled from 23 poultry farms

following two visits, usually 4weeks apart, to each farm.

From 14 shopping malls studied, each visited twice, a total

of 31 egg sample composites, with 6 eggs pooled to consti-

tute a composite while from 102 other retailers visited once,

a total of 107 composite egg samples, with 6 eggs per com-

posite, were studied (Adesiyun et al., 2005b). Overall, for

the three sources studied a total of 1978 pooled table eggs

sample were cultured for bacteria.

2.2. Swab sampling of eggs

For 25 pooled eggs sampled from each farm following

each visit, a sterile swab each soaked in saline was applied

to the surface of each egg shell and the 25 swabs were then

dipped into 20 ml of saline. For 6 pooled eggs from each

outlet of either the shopping malls or supermarkets, each

egg shell surface was similarly swabbed with a sterile swab

and then dipped in 6 ml of saline. The contents of the saline

were mixed thoroughly using a vortex mixer (Henry Treem-

ner, USA) and subsequently inoculated into appropriate

bacteriological media.

2.3. Sampling of egg contents

Each egg in the pool of 25 (farms) or 6 (malls and other

retailers) was dipped in 95% ethanol for 5min after which

the pointed end of each was Xamed for 510 s. with a Bun-

sen burner after which a sterile scalpel blade was used to

make a small hole on the shell through which the contentswere emptied into a stomacher bag. The egg contents (yolk

and albumen) in each pool were then blended for 30s at

normal speed in a Stomacher 400 (Seaward, London,

United Kingdom) after which the mixture was used to

inoculate appropriate bacteriological media.

2.4. Isolation and identiWcation of bacteria

To isolate Salmonella 10 ml of egg contents or 1ml of

saline containing swabs were added to 90ml of lactose

broth and incubated at 37 C overnight as pre-enrichment.

Enrichment, isolation and identiWcation ofSalmonella fol-

lowed standard methods (FAO, 1992). Salmonella polyva-

lent antiserum A-I & Vi (Difco, Michigan, USA) was used

to initially serologically identify Salmonella by slide aggluti-

nation test.

To detect E. coli, swabs of egg shells and egg contents

were used to inoculate eosin methylene blue (EMB) and

identiWed using standard methods (Macfaddin, 1977).

2.5. Determination of the antimicrobial sensitivity

of Salmonella and E. coli isolates

To determine the antimicrobial sensitivity of the iso-

lates, the NCCLS (2002) protocol was used with the fol-lowing antimicrobial agents on discs (Difco, Michigan,

USA) and concentrations: kanamycin (30mcg), streptomy-

cin (10mcg), nalidixic acid (30 mcg), gentamicin (10 mcg),

tetracycline (30 mcg), sulphamethoxazole/trimethoprim

(23.25/1.75 mcg) and enroXoxacin (5mcg). The tests were

performed on Mueller Hinton agar (Difo, Michigan, USA)

followed by incubation aerobically at 35C for 24 h. The

zone of inhibition were interpreted as recommended by the

disc manufacturer and the NCCLS (2002).

2.6. Characterization of bacteria

All Salmonella isolates were kindly conWrmed and sero-

typed by the Caribbean Epidemiology Centre (CAREC),

Port of Spain, the regional reference centre for Salmonella.

Representatives (19) ofSalmonella Enteritidis isolates from

each source were kindly phage typed by CAREC and the

Veterinary Laboratories Agency, Weybridge, United King-

dom (Ohare et al., 2004). To determine phenotypic charac-

teristics of all E. coli isolates, they were inoculated onto

blood agar and sorbitol MacConkey agar and incubated at

37 C overnight. The ability of E. coliisolates to produce

mucoid and haemolytic colonies was determined on blood

agar while the ability to utilize sorbitol was determined on

sorbitol MacConkey agar. Isolates ofE. coliwere screened


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308 A. Adesiyun et al. / Food Control 18 (2007) 306311

for O157 strain by the use ofE. coliO157 antiserum (Difco,

Michigan, USA) by the slide agglutination test.

2.7. Statistical analysis

The diVerences in the frequency of sensitivity to the vari-

ous antimicrobial agents by the isolates ofSalmonella andE. coli, source of eggs and location in eggs were compared

and determined to be statistically signiWcant using the chi-

square tests using the Statistical Package for Social

Sciences (SPSS) version 10. All statistical analyses were

two-tailed and interpreted at the 5% level of signiWcance.

3. Results

3.1. Frequency of antimicrobial sensitivity amongst isolates

of Salmonella and E. coli

Of a total of 74 isolates of Salmonella spp. tested, 17

(22.9%) were resistant to one or more of the seven antimi-

crobial agents compared with 104 (88.1%) of 118 isolates of

E. coli(Table 1). The diVerence was statistically signiWcant

(P < 0.05; X2). For both bacteria, resistance was compara-

tively high to streptomycin (54.2%) and tetracycline (35.9%)

but low to gentamicin (11.5%) and sulphamethoxazole/

trimethoprim (9.4%).

3.2. Frequency of resistance patterns amongst isolates

Only 1 (1.4%) isolate ofSalmonella spp. was multi-resis-

tant, i.e. resistant to 3 or more antimicrobial agents com-

pared with as many as 55 (46.6%) of 118 isolates ofE. colitested. The predominant resistance patterns amongst E. coli

isolates were: S-TE (20 isolates), S (17 isolates), S-TE-K (13

isolates), SXT-ENR-S-NA-TE-K (7 isolates) and SXT-S-

TE (5 isolates).

3.3. Distribution of resistance amongst Salmonella serotypes

Amongst the eight serotypes of Salmonella tested, 6

(66.7%) exhibited resistance to antimicrobial agents. Eight

(19.5%) of 41 isolates ofS. Enteritidis were resistant while 4

(26.7%) of 15 S. Ohio isolates exhibited resistance (Table 2).

The resistance patterns observed were as follows: for S.Enteritidis: NA (4 isolates), S (2 isolates), K (1 isolate) and

CN-S-K (1 isolate); S. Ohio: S (3 isolates) and NA (1 iso-

late); S. Javiana (2 isolates); S. Braenderup: S (1 isolate);

S. Georgia: S (1 isolate) and S. Caracas (1 isolate).

3.4. Relationship between frequency of resistance amongst

Salmonella and E. coli isolates and location of isolation

from eggs

The diVerences in the frequency of resistance to antimi-

crobial agents amongst the isolates of bacteria recovered

from the shells and contents of table eggs were not statisti-

cally signiWcant (P > 0.05; X2) as shown in Table 3.

3.5. Distribution of resistant strains of Salmonella and

E. coli by source of egg

For both bacteria, the frequency of resistance to antimi-

crobial agents was highest Amongst isolates from shoppingmalls but the diVerence was not statistically signiWcant

(P > 0.05; X2) compared to isolates from either layer farms

or other retailers (Table 4).

3.6. Phage type of S. Enteritidis isolates

All 19 isolates ofS. Enteritidis representive of all sources

belonged to phage type 1 (PT1).

3.7. Phenotypic characteristics of E. coli

All 118 isolates of E. coli were non-haemolytic, non-mucoid, sorbitol-fermenters and non-O157 strains.

Table 1

Frequency of resistance to antimicrobial agents amongst isolates ofSalmonella and Escherichia coli

a From eggs shell, albumen or yolk of egg.b Resistant to one or more of the seven antimicrobial agents tested.c Sstreptomycin, TEtetracycine, Kkanamycin, NAnalidixic acid; ENRenroXoxacin; CNgentamicin, SXTsulphamethoxazole/trimetho-

prim.

Type of bacteria No. of isolates

testedaNo. (%) of isolate

resistantb

c S TE K NA ENR CN SXT

Salmonella spp. 74 17 (22.9) 11 (14.9) 0 (0.0) 2 (2.7) 5 (6.8) 0 (0.0) 1 (1.4) 0 (0.0)

E. coli 118 104 (88.1) 93 (78.8) 69 (58.5) 38 (32.2) 31 (26.3) 30 (25.4) 11 (9.3) 18 (15.3)

Total 192 121 (63.0%) 104 (54.2) 69 (35.9) 40 (20.8) 36 (18.8) 30 (15.6) 12 (6.3) 18 (9.4)

Ta le 2

Frequency of resistance to antimicrobial agents by serotypes of Salmo-

nella

a Resistant to one or more antimicrobial agents.

Serotypes ofSalmonella No. of isolates tested No. (%) resistancea

Enteritidis 41 8 (19.5)

Ohio 15 4 (26.7)

Mbandaka 6 0 (0.0)

Javiana 5 2 (40.0)

Braenderup 3 1 (33.3)

Group C1 2 0 (0.0)

Georgia 1 1 (25.0)

Caracas 1 1 (25.0)

Total 74 17 (22.9)


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4. Discussion

A prevalence of 22.9% for resistance to antimicrobial

agents detected amongst Salmonella isolates from eggs is

considerably lower than the prevalence of 50% reported forSalmonella isolates recovered from livestock in Trinidad

(Adesiyun, Kaminjolo, Loregnard, & Kitson-Piggott, 1993)

and elsewhere (Brown, Baggesen, Hansen, Hansen, & Bisg-

aard, 1994; Hangombe et al., 1999; Poppe, 1994; Neu,

Cherubin, Longo, Flouton, & Winter, 1975; Wray, Beedell,

& McLaren, 1991). Amongst S. Enteritidis isolates, the

prevalence of multi-resistance was considerably low, 1.4%,

a Wnding in agreement with report ofYang et al. (2002) but

considerably lower than that found by Dias de Oliveira,

Siqueira Flores, Dos Santos, and Brandelli (2005). Simi-

larly, the prevalence of resistance, as determined by resis-

tance to one or more antimicrobial agents, although higher

than reported for isolates from table eggs by Brown et al.

(1994) is signiWcantly lower than found in Brazil (Simango

& Mbewe, 2000). It has been reported that inappropriate

use of antimicrobial agents in livestock may result in the

development of resistance amongst bacteria in these ani-

mals or their products (Washington, 1979; Waltner-Toews

& McEwen, 1994).

The prevalence of 88.1% for resistance to antimicrobial

agents amongst E. coliisolates from table eggs is compara-

ble to the prevalence of E. coli isolated from livestock,

89.7% (Adesiyun & Kaminjolo, 1992). The generally high

prevalence of resistance to antimicrobial agents has been

attributed to uncontrolled use of antimicrobial agents as

growth promoters or in the treatment of bacterial infec-

tions with the farmers having unlimited access to these

agents and their use (Adesiyun & Kaminjolo, 1992). The

report on the detection of antimicrobial agents (sulfona-

mides, macrolides and tetracyclines) in table eggs oVeredfor sale in Trinidad was therefore not a surprise (Adesiyun

et al., 2005a).

The detection that exhibition of resistance was highest to

streptomycin amongst all enteric bacteria studied is similar

to the Wndings for other bacteria from several sources in the

country attributed to the widespread use of the antimicro-

bial agents in several preparations available locally

(Adesiyun & Kaminjolo, 1992; Adesiyun et al., 1993). Fur-

thermore, on the layer farms in Trinidad it has been

reported that as many as 87% of the farms admitted to the

use of medicated feeds that contained chlortetracycline, tyl-

osin or nitrofurantoin (Adesiyun et al., 2005a). Unlike in

other studies where Salmonella, including S. Enteritidis,

have been reported to be sensitive to nalidixic acid, kana-

mycin and gentamicin (Dias de Oliveira et al., 2005;

Hangombe et al., 1999), in the present study, together with

resistance exhibited to streptomycin, the isolates were also

resistant to the three antimicrobial agents albeit, at a

relatively low prevalence.

It was hardly a surprise that no statistically signiWcant

diVerences were observed in the prevalence of resistance

amongst bacteria obtained from the three sources (farms,

shopping malls and other retailers). This is so because a

majority of table eggs sampled from the sale outlets (malls

and other retailers) actually originated from most of the 23

Ta le 3

Frequency of resistance to antimicrobial agents amongst Salmonella and E. coliisolates from egg shell and yolk/albumen (content)

a A mixture of yolk and albumen.b Resistant to one or more antimicrobial agents.

Type of bacteria Location of isolation from eggs

Shell Albumen/Yolka

No. of isolates tested No. (%) of isolates resistantb No. of isolates tested No. (%) of isolates resistantb

Salmonella spp. 28 7 (25.0) 46 10 (21.7)

E. coli 88 76 (86.4) 30 28 (93.3)

Total 116 83 (71.6) 76 38 (50.0)

Ta le 4

Frequency of resistance to antimicrobial agents amongst Salmonella and E. coliisolates by sale outlet of table eggs

a Layer farms studied from where vendors could purchase eggs directly.b Sale outlets in shopping malls across the country where all eggs are stored refrigerated.c Outlets for eggs which include kiosks, roadside vendors, supermarkets (small, medium or large) selling eggs where few had refrigerated facilities.d Resistant to one or more antimicrobial agents.

Type of bacteria Sale outlet

Farmsa Mallsb Other retailersc

No. of isolates

tested

No. (%) of isolates

resistantdNo. of isolates

tested

No. (%) of isolates

resistantcNo. of isolates

tested

No. (%) of isolates

resistantc

Salmonella spp. 18 3 (16.7) 25 8 (32.0) 31 6 (19.4)

E. coli 60 52 (86.7) 23 21 (91.3) 35 31 (88.6)

Total 78 55 (70.5) 48 29 (60.4) 66 37 (56.1)


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