Techniques for the isolation of salmonellas from eggs

Horizontal transmission of Salmonella enteritidis in groups of experimentally infected laying hens housed in different housing systems

Concerns regarding the welfare of laying hens have led to the ban of conventional battery cages in Europe from 2012 onward and to the development of alternative housing systems that allow hens to perform a broader range of natural behaviors. Limited information is available about the consequences of alternative housing systems on transmission of zoonotic pathogens such as Salmonella Enteritidis. However, Salmonella enterica serovar Enteritidis continues to be one of the leading causes of bacterial foodborne disease worldwide and this is mainly attributed to the consumption of contaminated eggs. A transmission experiment was performed to quantify the effect of the housing system on the spread of a Salmonella Enteritidis infection within a group of layers and on internal egg contamination. At the age of 16 wk, 126 birds housed on the floor were inoculated with Salmonella Enteritidis. Three weeks later, the inoculated hens were housed together with equal numbers of susceptible contact animals in 4 different housing systems: a conventional cage system, a furnished cage, an aviary, and a floor system. Transmission and egg contamination were followed during a 4-wk period. A trend toward increased bird-to-bird transmission was detected in the aviary and floor system compared with the cage systems. Also, significantly more contaminated eggs were found in the aviary compared with the cage systems and the floor system. These results suggest that there is a clear need to optimize and maintain Salmonella surveillance programs when laying hens will be moved from conventional cage systems to alternative housing systems.

Mechanisms of egg contamination by Salmonella Enteritidis

Salmonella Enteritidis (SE) has been the major cause of the food-borne salmonellosis pandemic in humans over the last 20 years, during which contaminated hen's eggs were the most important vehicle of the infection. Eggs can be contaminated on the outer shell surface and internally. Internal contamination can be the result of penetration through the eggshell or by direct contamination of egg contents before oviposition, originating from infection of the reproductive organs. Once inside the egg, the bacteria need to cope with antimicrobial factors in the albumen and vitelline membrane before migration to the yolk can occur. It would seem that serotype Enteritidis has intrinsic characteristics that allow an epidemiological association with hen eggs that are still undefined. There are indications that SE survives the attacks with the help of antimicrobial molecules during the formation of the egg in the hen's oviduct and inside the egg. This appears to require a unique combination of genes encoding for improved cell wall protection and repairing cellular and molecular damage, among others.

Optimization of iron supplementation for enhanced detection of Salmonella Enteritidis in eggs

Mixed raw egg contents were inoculated with approximately 10 CFU of Salmonella Enteritidis and supplemented with 0 to 7 mg of FeSO4 per g of egg contents. Egg contents were then incubated at 37 degrees C, and Salmonella Enteritidis colonies were enumerated for up to 106 h. Iron supplementation significantly enhanced the growth of Salmonella Enteritidis. Within the first 24 h of incubation, the optimum iron level for Salmonella Enteritidis growth in egg contents was between 0.2 and 2 mg of FeSO4 per g of egg contents. After 24 h of incubation at 37 degrees C. Salmonella Enteritidis counts in eggs supplemented with 0.5 mg of FeSO4 per g of egg contents consistently reached approximately 1 x 10(9) CFU/ml, whereas Salmonella Enteritidis counts in eggs without iron supplementation varied from less than 5 CFU/ml to 8.4 x 10(6) CFU/ml. A 3 by 3 factorial design was used to study the effect of type of preenrichment and level of iron supplementation on the growth of Salmonella Enteritidis in egg contents. No significant differences in Salmonella Enteritidis counts between preenrichment and nonpreenrichment treatments were observed when egg contents were supplemented with 0.5 mg of FeSO4 per g of egg contents. It was concluded that preenrichment was not necessary for isolation of Salmonella Enteritidis from eggs. The effect of iron supplementation on the sensitivity of detection by the direct plating method was investigated. The direct plating method detected a significantly higher percentage of Salmonella Enteritidis in raw egg contents supplemented with 0.5 mg of FeSO4 per g of egg contents (90%) than in raw egg contents without iron supplementation (63.3%).

Detection of Salmonella enteritidis in shell and liquid eggs using enrichment and plating

Detection methods using various enrichment and plating media and immunoconcentration for Salmonella enteritidis in shell and liquid eggs were evaluated. For liquid egg samples naturally contaminated with S. enteritidis, pre-enrichment in 225 ml of buffered peptone water with cysteine followed by selective enrichment in 10 ml of tetrathionate broth was the superior, resulting in the detection of S. enteritidis in all samples on six of the seven types of selective agar substrate investigated. This enrichment procedure also enabled detection of S. enteritidis in most of artificially inoculated shell egg and pasteurized liquid egg samples.

Infection model for hatching chicks infected with Salmonella enteritidis

A total of six different strains of Salmonella enteritidis (phage type 4) were used for the artificial infection of the egg yolk and albumen of SPF hatching eggs. An artificial yolk infection with low infection doses of S. enteritidis resulted in a considerable multiplication of salmonellas in the egg after a short time and a rapid embryonic death. After inoculation of 10(3) cfu of different S. enteritidis strains into the albumen, living day-old chicks infected with salmonellas were hatched. This model corresponds to a primary vertical transmission of salmonellas and permits the investigation of problems concerning the pathogen/host relationship.

Salmonella enteritidis: the egg and I

The world-wide clinical incidence of Salmonella Enteritidis has increased markedly. The increase is associated with the enhanced ability of the bacterium to systemically colonise layer chickens. Subsequent contamination and consumption of intact shell eggs from colonised layer hens, either directly or in foods containing raw or lightly cooked eggs, causes human disease. Despite investigation, no change in the biology of the bacterium has been correlated with increased colonisation in chickens. To date, no method of control at the production level has proven effective; consumer education is the best means of minimising the public health risk.

Evaluation of a rapid cultural method for identification of salmonellas in naturally contaminated veterinary samples

Three thousand and forty-two samples of post-mortem tissues, environmental swabs, animal feed ingredients and eggs were examined by conventional methods and by a simple rapid culture method. The rapid method gave reliable presumptive identification of salmonella contamination within 48 h of the start of culture. The rapid method also showed enhanced sensitivity for detection of salmonellas, in most types of sample, particularly those taken in poultry houses and hatcheries where high levels of competitive faecal bacteria are present.

Contamination of egg shell and contents with Salmonella enteritidis: a review

Salmonella enteritidis can contaminate the contents of clean, intact shell eggs as a result of infections of the reproductive tissue of laying hens. The principal site of infection would appear to be the upper oviduct. In egg contents the most important sites of contamination are either the outside of the vitelline membrane or the albumen surrounding it. In fresh eggs, only few salmonellas are present and as albumen is an iron-restricted environment, growth will only occur once storage-related changes to vitelline membrane permeability, which allow salmonellas to invade yolk contents, have taken place. When this happens high populations are achieved in both yolk contents and albumen. Some eggs from naturally infected hens have been found to contain large numbers of S. enteritidis. The rate of change in membrane permeability is temperature-dependent. In eggs stored at 20 degrees C, yolk invasion is uncommon until eggs have been stored for 3 weeks. In stimulated kitchen conditions where temperatures reached 30 degrees C, salmonellas could grow rapidly after a few days.

Conventional methods for the detection and isolation of Salmonella enteritidis

Conventional methods for the specific isolation of Salmonella enteritidis are scarce. For pre-enrichment, addition of ammonium-iron (III)-citrate, ferrioxamine E and G or novobiocin in combination with cefsoludin to Buffered Peptone Water (BPW) and of ferrous sulphate to Trypticase Soy Broth seems to favour S. enteritidis isolation. As far as selective media are concerned, the use of semi-solid media, and addition of nitrofurantoin, results in higher isolation rates of the S. enteritidis serovar. Addition of 0.0015% nitrofurantoin to semi-solid DIASALM is so far the only successful combination reported. Addition of 0.0015% nitrofurantoin to solid media, in this case to XLD, is also reported. Use of a semi-solid medium, preferably DIASALM + 0.0015% nitrofurantoin, in addition to the selective media routinely used is recommended.

Research note: evaluation of direct plating for detecting Salmonella enteritidis in pools of egg contents

The effectiveness of direct plating of incubated egg contents pools onto selective agar media for detecting contamination by Salmonella enteritidis (SE) was evaluated in comparison to several broth enrichment strategies. Pools of mixed liquid whole egg were inoculated with very small number of SE cells, incubated for 4 days at 25 C, and sampled by direct plating or both enrichment culture. Methods involving broth nonselective (pre-enrichment) and selective broth culture steps detected a significantly higher percentage of contaminated egg pools than did direct plating, but required 48 h longer to provide results. Methods involving direct selective broth enrichment, without prior pre-enrichment, did not perform significantly better than direct plating. Direct plating appears to provide a relatively rapid and inexpensive method for detecting SE in egg pools, although greater sensitivity is attainable with more intensive methods.