Population structure, origins and evolution of major Salmonella enterica clones

The genus Salmonella consists of two species S. enterica and S. bongori. S. enterica has a well defined subspecies structure with seven subspecies consistently delineated by sequence variation. Frequency of recombination between subspecies and within a subspecies is markedly different. Subspecies I undergoes frequent recombination as demonstrated recently, demystifying the long-held belief that Salmonella is a highly clonal organism. The majority of disease causing serovars are from subspecies I with the most important serovars in human health being Typhimurium and Typhi. Typhimurium has developed considerable diversity and may be a very old serovar. The majority of the isolates belong to a single clonal complex by multilocus sequence typing. Typhimurium isolates are divided into phage types and some of the phage types do not have a single origin as determined using mutational changes. Phage type DT104 is heterogeneous and represented in multiple sequence types, with its multidrug-resistant variant most successful causing epidemics in many parts of the world. Typhi, a human restricted serovar, is relatively young compared to Typhimurium, and has a low level of sequence variation. Single nucleotide polymorphisms (SNPs) have been shown to be very useful for typing and resolving relationships within Typhi. Genome sequences of 19 isolates revealed more than 1700 SNPs. The fully resolved phylogenetic tree allows one to trace the mutational changes occurred during clonal diversification. Genome wide SNPs have greatly enhanced our understanding of the evolution of Salmonella clones.     

A study on prevalence of multi-drug-resistant (MDR) Salmonella in water sprinkled on fresh vegetables in Bareilly, Moradabad, and Kanpur (northern Indian cities)

Water-borne and food-borne diseases are common in summers and monsoons in India. This study between March 2004 to September 2004 on microbiological quality of water used by vegetable vendors to keep their vegetables fresh was conducted to asses the role of water as a source of Salmonella. Of the 309 samples collected from Bareilly (80 vegetable vendors, 47 ponds, five municipal water taps), Moradabad (74 vegetable vendors, three ponds, five municipal water taps) and Kanpur (84 vegetable vendors, six ponds, five municipal water taps), 82 (26.5%) and 27 (8.7%) had Escherichia coli and Salmonella enterica strains, respectively. The study revealed that sprinkling water was contaminated with 0.00–6.81 log₁₀ cfu ml⁻¹ of coliforms, 4.16 log₁₀ to 9.46 log₁₀ cfu ml⁻¹ of heterotrophic aerobic bacteria, 0.00 to 7.23 log₁₀ cfu ml⁻¹ of non-lactose fermenters, 0.00 to 5.56 log₁₀ cfu ml⁻¹ of Salmonella and 0.00 to 7.77 log₁₀ cfu ml⁻¹ of yeast and moulds. Similarly, microbial counts in pond water samples (Bareilly) were 0.00 to 6.06 log₁₀ cfu ml⁻¹ of faecal coliforms, 5.12 log₁₀ to 8.09 log₁₀ cfu ml⁻¹ of heterotrophic aerobic bacteria, 0.00 to 6.37 log₁₀ cfu ml⁻¹ of non-lactose fermenters, 0.00 to 5.73 log₁₀ cfu ml⁻¹ of Salmonella and 0.00 to 7.82 cfu log₁₀ ml⁻¹ of yeasts and moulds. Presence of Salmonella in water sample had negative correlation with number of coliforms and positive correlation with number of non-lactose fermenters, as of the 16 (6.7%) Salmonella positive samples of water from vegetable vendors, ten were negative for coliforms. Similarly, of the 11 pond-water samples positive for Salmonella, six were negative for coliforms, and negative correlation (−0.55) between coliform count and Salmonella was statistically significant (r_0.01). On the other hand, Salmonella counts could be positively correlated (r_0.01) with counts of non-lactose fermenters. Salmonella isolates from water for sprinkling on vegetables belonged to S. Anatum (1), S. Newport (1), S. Saintpaul (6), S. Virchow (4) and S. Weltevreden (4) serovars while isolates pond water samples belonged to S. Saintpaul (9) and S. Newport (2) serovars. Except two Salmonella isolates (one each of serovar S. Anatum and S. Weltevreden), all had multiple drug resistance and could be classified into 21 resistotypes. All the Salmonella isolates were sensitive to ceftriaxone and streptomycin while resistant to sulphamethizole. The study indicated that pond water (used by farmers for washing vegetables) and water used by vegetable vendors for sprinkling on vegetables might have an important role as a source of multiple-drug-resistant zoonotic Salmonella.     

Salmonella serovars isolated for the first time in Poland, 1995–2007

Objectives

Almost each year, a few Salmonella serovars isolated for the first time are noted in Poland. Each such serovar should be regarded as potentially dangerous. Recognition of them and monitoring their occurrence and frequency of distribution in all sources is important to control salmonellosis, and is one of the roles of the National Salmonella Centre.

Materials and Methods

Over 2000 Salmonella strains submitted to the National Salmonella Centre for reference identification were examined. All of them were isolated in Poland between 1995?C2007 from human and non-human sources. They were serotyped and characterized biochemically according to standard techniques. The antigenic factors were identified by means of good-quality Salmonella specific rabbit antisera. The White-Kauffmann-Le Minor scheme was used to name the serovars.

Results

One hundred and forty-five serovars were recognized. Fifty-two of them appeared for the first time in Poland. One serovar was found to be the new one. Its validation was done at the WHO Collaborating Centre for Reference and Research on Salmonella (Institut Pasteur, Paris, France). It will be included in the next (10th) edition of the White-Kauffmann-Le Minor scheme.

Conclusions

Salmonella will continue to be the feature of humans, animals and the general environment. For practical purposes, we have to accept that Salmonella will not be eliminated and we must address our efforts to its controlling. Measuring trends in Salmonella serovars over time ought to be done to provide information about the efficacy of prevention and control measures. Results of this study are significant for these activities. They have also allowed to update the list of Salmonella serovars isolated in Poland during the last fifty years (1957?C2007) which is presented as well.     

Gonococcal infection within Scotland: Antigenic heterogeneity and antibiotic susceptibility of infecting strains

Two panels of monoclonal antibody reagents were used to serotype all strains of Neisseria gonorrhoeae isolated from four separate geographical areas serving two million of the five million Scottish population. Serotype 113 isolates accounted for 60% of the 869 strains examined and were more prevalent than 1A isolates in each geographical area. A total of 11 JA serovars and 47 1B serovars were recognised. Only two of the 11 IA serovars (Aedgkih/Arost and Aedih/Arst) were found in every centre but these accounted for over 90% of the IA isolates. Although there was a total of 47 different 113 serovars over 80% of the isolates were accounted for by the ten most commonly encountered serovars. There were, however, marked geographical differences within both major and minor serovars. There was a highly significant difference (P < 0.001) between protein IA and 1B serovars with respect to their susceptibility to penicillin. Within each protein 1 type there were also differences in antibiotic susceptibility. Penicillinase-producing N. gonorrhoeae (PPNG) were found in all centres and accounted for 24 (2.8%) of the 869 isolates. The majority of the PPNG (71%) were serotype IA and with one exception were serovar Aedih/Arst. PPNG strains accounted for 37% (16) of the 43 Aedih/Arst isolates. Epidemiological, diagnostic and therapeutic implications arising from the distinct geographical differences in the pool of circulating gonococci are discussed.     

Current status of serotyping of Neisseria gonorrhoeae

Protein I has become the basis of serotyping of N. gonorrhoeae. A panel of 12 anti-protein I monoclonal antibodies is supplied by Syva Company, Palo Alto, CA, through distributors for research purposes only. These distributors are responsible for storing the monoclonal antibodies, supplying research groups and training new users, particularly in the interpretation of coagglutination reactions. A European Workshop was held in Heidelberg, Federal Republic of Germany in July 1988 to discuss serotyping with these antibodies. The current uses of serotyping of N. gonorrhoeae include epidemiological studies, clinical purposes and surveillance of antibiotic resistance and plasmid carriage. Predominant serovars may be subtyped either by additional antibodies or with the use of another technique, such as auxotyping, determination of antibiotic sensitivities, plasmid analysis or genetic fingerprinting. Conversely, there is growing evidence that it may be appropriate to group certain serovars together. Data collected during prevalence studies could be combined with clinical information and an international data bank set up. This supposes a close future collaboration of all groups involved in the epidemiology of N. gonorrhoeae.Corresponding author.     

Comparative genomic analysis between typhoidal and non-typhoidal Salmonella serovars reveals typhoid-specific protein families

BackgroundThe genus Salmonella contains more than 2600 serovars. While most cause a self-limiting gastroenteritis, four serovars, S. Typhi, S. Paratyphi A, B and C, elicit typhoid, a potentially fatal systemic infection. Because of the prevalence in certain regions, such as South Asia, and the disease severity of typhoidal Salmonella infections, comprehensive studies are needed to elucidate the pathogenesis of diseases caused by these typhoidal serovars.ResultsWe performed comparative genomic analyses on eight human typhoidal strains and 27 non-human typhoidal Salmonella strains to elucidate their evolutionary relationships and identify the genes specific to the four typhoidal serovars. Our results indicate that Salmonella may have an open pan-genome. A core-genome based phylogeny demonstrated that divergence between S. Paratyphi A and S. Typhi took place not long ago and S. Paratyphi B shared a recent common ancestor with S. Paratyphi C. Of great interest, the divergence between S. Paratyphi B and S. Paratyphi C was shown to be more recent than that between S. Paratyphi A and S. Typhi. Alignment and comparisons of the genomes identified unique complements of protein families to each of the typhoidal serovars. Most of these protein families are phage related and some are candidate virulence factors. Importantly, we found 88 protein families specific to two to three of the four typhoidal serovars. All but two of the 88 genes are present in S. Typhi, with a few in the three paratyphoidal serovars but none in the non-human typhoidal serovars. Most of these genes are predicted to encode hypothetical proteins and some are known to code for virulence factors such as Vi polysaccharide related proteins.ConclusionsBy comprehensive genomic comparisons, we identified protein families specific to the human typhoidal serovars, which will greatly facilitate investigations on typhoid pathogenesis.     

Results of the three-year surveillance by the Italian SALM-NET System: Human isolates of Salmonella serotypes

Within the wide framework of the European Community Human Salmonella Surveillance Project (SALM-NET), the data on the most commonly isolated serotypes in Italy from January 1994 to December 1996 are reported. The total data included in the Italian SALM-NET data base account for 34,412 Salmonella isolates. In the list of the most frequent isolates, S. enteritidis ranked always first in the years 1994, 1995 and 1996 with 5435 (43.4%), 4589 (37.1%) and 4044 (42.4%), respectively, over the total number of Salmonella isolates. This serotype is followed by S. typhimurium in the list of the top ten isolates, with 2236 (17.9%), 2831 (22.9%) and 2239 (23.5%). The other serotypes included in this list accounted for a much lower number and percentages ranging from 505 isolates of S. derby (4.0%) to 99 isolates of S. brandenburg (0.8%).     

一起食源性疾病暴发事件分离的2种血清型沙门菌的病原学分析

目的 对一起食源性疾病暴发事件中分离的2种血清型沙门菌进行病原学分析。方法 采集2022年9月8日某学校暴发事件中病例肛拭子11件、可疑污染食品13件和环境样本10件；对病例肛拭子分别使用亚硒酸盐煌绿增菌液和脑心浸液肉汤（BHI）进行增菌培养，在完成常见肠道病原菌荧光PCR检测后，根据结果开展相应致病菌的分离培养；对可疑污染食品参照食品安全国家标准进行检测；每份肛拭子样本和食品样本均挑取多个疑似沙门菌菌落进行血清学凝集和全基因组测序；根据全基因组测序结果确定沙门菌血清型，基于菌株核心基因组单核苷酸多态性（SNP）进行聚类分析。结果 病例肛拭子和可疑污染食品沙门菌检出率分别为9/11和5/13，其中4例病例肛拭子和4件可疑污染食品样本均分离到2种血清型沙门菌（乌干达沙门菌和伊迪坎沙门菌），其余阳性样本分离沙门菌均为单一乌干达沙门菌血清型或单一伊迪坎沙门菌血清型。11件病例肛拭子样本接种BHI增菌液后12 h和24 h沙门菌荧光PCR检出率均为9/11，与分离培养结果一致。2种血清型沙门菌在基于核心基因组SNP构建的聚类树中形成2个相互独立且遗传距离较远的分支，而每一种血清型沙门菌也表现出基因组多态性，乌干达沙门菌之间SNP差异个数介于0~14个，伊迪坎沙门菌之间SNP差异个数介于0~23个。结论 本次事件为乌干达沙门菌和伊迪坎沙门菌共同导致食源性疾病暴发事件，基于BHI对病例肛拭子增菌并进行沙门菌荧光PCR检测的方案可在暴发中应用。     

Survival of Salmonella spp. and fecal indicator bacteria in Vietnamese biogas digesters receiving pig slurry

Small-scale biogas digesters are widely promoted worldwide as a sustainable technology to manage livestock manure. In Vietnam, pig slurry is commonly applied to biogas digesters for production of gas for electricity and cooking with the effluent being used to fertilize field crops, vegetables and fish ponds. Slurry may contain a variety of zoonotic pathogens, e.g. Salmonella spp., which are able to cause disease in humans either through direct contact with slurry or by fecal contamination of water and foods. The objective of this study was to evaluate the survival of Salmonella spp. and the fecal indicator bacteria, enterococci, E. coli, and spores of Clostridium perfringens in biogas digesters operated by small-scale Vietnamese pig farmers. The serovar and antimicrobial susceptibility of the Salmonella spp. isolated were also established. The study was conducted in 12 farms (6 farms with and 6 farms without toilet connected) located in Hanam province, Vietnam. Sampling of pig slurry and biogas effluent was done during two seasons. Results showed that the concentration of enterococci, E. coli, and Clostridium perfringens spores was overall reduced by only 1–2 log₁₀-units in the biogas digesters when comparing raw slurry and biogas effluent. Salmonella spp. was found in both raw slurry and biogas effluent. A total of 19 Salmonella serovars were identified, with the main serovars being Salmonella Typhimurium (55/138), Salmonella enterica serovar 4,[5],12:i:- (19/138), Salmonella Weltevreden (9/138) and Salmonella Rissen (9/138). The Salmonella serovars showed similar antimicrobial resistance patterns to those previously reported from Vietnam. When promoting biogas, farmers should be made aware that effluent should only be used as fertilizer for crops not consumed raw and that indiscriminate discharge of effluent are likely to contaminate water recipients, e.g. drinking water sources, with pathogens. Relevant authorities should promote safe animal manure management practices to farmers and regulations be updated to ensure food safety and public health.     

A population-based longitudinal study on the incidence and disease burden of gastroenteritis and Campylobacter and Salmonella infection in four regions of the Netherlands

The aim of this study was to estimate the incidence of gastroenteritis and Campylobacterand Salmonellainfection in the Dutch population, the disease burden, and the percentage of patients with gastroenteritis that consults a general practitioner. A sample of 6243 persons was invited to participate in the study, i.e. completing a questionnaire and submitting stool samples. The follow-up period was 17 weeks. In total, 2206 persons participated (= 35%), contributing 660 person years. The incidence (standardised by age and gender) of first episodes of gastroenteritis was 45 per 100 person years. Among patients with gastroenteritis, Salmonellaand Campylobacterwere cultured in 1.6 and 4.5%, respectively. The standardised incidence of first Campylobacterinfections was 9 per 100 person years, of first Salmonellainfections 4 per 100 person years. For 22% of the episodes of gastroenteritis, a general practitioner was consulted (either by phone or by practice visit). For 52% of the episodes, medicine were used. For 34% of the episodes, absence from school was reported and for 15%, absence from work was reported. Despite of possible biases, we can conclude that the incidence of gastroenteritis is very high and causes considerable use of medication, consultation of general practitioners and absence from work and school.     

Molecular marker analysis ofSalmonella typhimurium from surface waters,humans, and animals

Salmonella contamination of North Sea water was detected for the first time in 1988 in Germany during routine examinations of bathing areas. Since then, subsequent isolations along the coast have been reported regularly. To define the source of contamination, strains isolated from seawater and rivers were studied by molecular marker methods. Their properties were compared with those of strains originating from possible sources of contamination such as humans, cattle, and sewage treatment plant water. Plasmid profile analysis of whole bacterial populations and the determination of antibiotic resistance patterns demonstrated, that contamination through the surrounding cattle industry could be excluded. Cattle isolates belonged to a widespread clone of phage type 204c which was multiresistant and exhibited an unique plasmid pattern which was never found in sea water isolates. Outer membrane protein and lipopolysaccharide analysis failed to demonstrate differences among theSalmonella populations and proved in this case insufficient for molecular marker discrimination.     

An outbreak of extended-spectrum, β-lactamase-producing Salmonella senftenberg in a burns ward

A strain of Salmonella senftenberg resistant to ceftazidime, gentamicin, chloramphenicol and ciprofloxacin was isolated from burn wounds of eight patients on a burns ward of a hospital in Dehli, India. The organism, which had probably been spread from patient to patient on staff hands, produced the extended-spectrum β-lactamase SHV-5 and the aminoglycoside-modifying enzymes AAC(3)II + AAC(6′). The strain was not isolated from stool cultures of any of the patients or staff, apart from the index patient who had a history of diarrhoea and fever before admission. The outbreak ended in three weeks, after the implementation of strict handwashing. This is the first report of SHV-5 β-lactamase in Salmonella spp. and also the first report of SHV-5 in India. The extended-spectrum β-lactamases that have been reported in Salmonella spp. now include the Group 2be enzymes SHV-2, SHV-5, TEM-3, TEM-25, TEM-27, CTX-M2, PER-1 and PER-2, and the Group 1 enzymes DHA-1 and CMY-2. The types of extended-spectrum β-lactamases produced by salmonellas, their association with aminoglycoside resistance and their geographical distribution are now similar to those seen in klebsiella. Increasing antibiotic resistance in these organisms is reducing therapeutic options for the treatment of invasive disease.     

Multi-epitope DnaK peptide vaccine against S.Typhi: An in silico approach

Salmonella is one of the key global causes of food and water borne enteric infections, responsible for significant morbidity and mortality worldwide especially in developing countries. Currently available vaccines against typhoid are moderately effective with several side effects and not efficacious against all Salmonella serovars. Due to limitations of these vaccines and emerging threats of multidrug resistance, developing an effective vaccine against these infections has increasingly become a priority. Heat shock proteins (Hsps), being evolutionarily conserved, represent dominant antigens in the host immune response. In continuation of our earlier studies on the development of S. Typhi DnaK and GroEL vaccine candidates, highly efficacious against Salmonella and multiple pathogens, in the present study, we have designed multi-epitope vaccine candidates common to multiple serovars of Salmonella using bioinformatics approach. Implementing various immunoinformatics tools such as IEDB, EpiJen, BCPRED, ElliPro and VaxiJen, led to the identification of many immunogenic B and T cell epitopes. The 3-D structure model of DnaK was generated to predict conformational B-cell epitopes using ElliPro server. Most promising T cell epitopes (29 CTLs, 18 T-helper cells) were selected based on their binding efficiency with commonly occurring MHC alleles. Finally we narrowed down to 5 protective antigenic peptides (PAPs), comprising highly conserved, antigenic and immunogenic B /T cell epitopes, least homologous with human host. These PAPs were predicted to be non-allergenic by allergenicity prediction tools (SORTALLER and AllerHunter). Hence, these immunogenic epitopes can be used for prophylactic or therapeutic usages specifically to defeat antibiotic-resistant Salmonella. These antigens have been reported for the first time and their conserved nature endow them as potential future vaccine candidates against other multiple pathogens as well.     

建设部门协同机制和监测网络应对沙门菌感染疫情

沙门菌病为常见的食源性疾病, 经常发生局部和跨地区的暴发流行, 随着全球经贸往来的日益频繁, 沙门菌跨境传播的风险也在不断增加。近期多国出现巧克力相关单相鼠伤寒沙门菌感染暴发疫情, 对我国食源性疾病特别是沙门菌病的监测和预警提出了更高的要求。加强临床机构与疾控和食品安全监测机构的合作、提升沙门菌基因组测序分析以及应用于流行病学监测和暴发溯源的能力、加强食品安全相关各部门间信息沟通及协作, 是及时发现暴发及传播、预防和控制传播的主要手段, 也是食源性疾病监测亟待加强的地方。     

Diagnostic potential of an iron-regulated hemin-binding protein HbpA that is widely conserved in Leptospira interrogans

Like most bacteria, Leptospira spp. requires iron for growth. However, they face conditions of iron limitation due to the low solubility of the ferric iron and additionally due to the observation that most of the available iron is held as protein-bound iron by the mammalian host. Our experimental observations with pathogenic Leptospira showed that they do not elaborate siderophores upon iron limitation. In Leptospira interrogans serovar Lai, we demonstrated direct acquisition of iron via an iron-regulated hemin-binding protein HbpA. PCR analysis and Southern hybridization studies revealed that the hbpA gene was conserved in the serovars belonging to L. interrogans species and was absent in the non-pathogenic Leptospira biflexa serovar Patoc and Leptospira meyeri serovar Ranarum. Here, we extended the PCR-based detection of the hbpA gene to clinical isolates and demonstrated the hbpA amplicon in all the serovars belonging to L. interrogans species from a total collection of 91 clinical isolates obtained from different geographical regions. In addition, we detected anti-HbpA antibodies in the serum of patients with leptospirosis. PCR-based detection of hbpA in clinical isolates of serovars and the in vivo expression of HbpA reflects the diagnostic potential of this antigen.     

基于微球液态阵列分子技术的沙门菌血清分型研究

目的 了解基于微球液态阵列分子技术对广东省腹泻病例沙门菌分离株的分型效果。方法 在微球液态阵列分子平台上应用SSA试剂盒对沙门菌人源株进行分子血清分型。结果 2010-2014年广东省人源沙门菌4 942株分为189种血清型,前100种血清型占所有菌株的98.08%（4 847/4 942）,其中98%可用SSA试剂盒完整分型;采用SSA试剂盒检测198株菌O抗原,可检出181株;用SSA检测H抗原的结果（98.32%,528/537）与传统血清凝集试验结果相符;fljB基因的符合率为93.09%（175/188）,fljB基因假阴性率为7.35%（9/134）,假阳性率为7.41%（4/54）;sdf基因和Vi基因符合率均为100%;用SSA试剂盒检测12株血清不能分型的沙门菌,有11株能成功分型。结论 SSA试剂盒能对96%以上的广东省人源沙门菌株进行分子血清分型,其结果与传统方法符合率超过98%。基于微球液态阵列分子技术的血清分型法比传统方法更具有高通量且快速的特点。     

A DIVA vaccine for cross-protection against Salmonella

Swine are often asymptomatic carriers of Salmonella spp., a leading cause of human bacterial foodborne disease. Vaccination against Salmonella is effective for protecting animal health and enhancing food safety. However, with >2500 Salmonella serovars, current vaccines for swine offer limited cross-protection against heterologous serovars. Also, existing vaccines can interfere with surveillance programs that monitor the Salmonella status of swine herds. To overcome Salmonella vaccine limitations, we rationally designed and constructed an attenuated Salmonella enterica serovar Typhimurium vaccine (BBS 866) by deleting multiple small regulatory RNA (sRNA) genes (omrA, omrB, rybB, micA, and invR) in combination with an rfaH mutation. We vaccinated swine intranasally at 3-weeks of age with PBS (mock-vaccinated), BBS 866 or BBS 202 (S. Typhimurium rfaH, Bearson et al., Front Vet Sci 2014;1:9.) and challenged at 7-weeks of age with virulent S. Choleraesuis, a swine pathogen. Vaccination with BBS 866 enhanced protection against S. Choleraesuis by significantly limiting the duration of fever, weight loss, the levels of circulating INFγ, and the total number of swine with S. Choleraesuis septicemia. Vaccination with either BBS 866 or BBS 202 significantly reduced S. Choleraesuis colonization of both systemic (spleen and liver) and gastrointestinal (Peyer's Patch, Ileocecal lymph nodes, and cecum) tissues. Similar to our earlier report for BBS 202, the BBS 866 vaccine strain can be used in swine without compromising the differentiation of infected from vaccinated animals (DIVA). Therefore, the attenuated S. Typhimurium BBS 866 strain, containing mutations in rfaH and multiple sRNAs, addresses the limitations of current Salmonella vaccines by providing cross-protection against Salmonella serovars in swine without interfering with established monitoring programs for Salmonella surveillance.     

Induction of a homologous and heterologous invasion–inhibition effect after administration of Salmonella strains to newly hatched chicks

Administration of live Salmonella strains to day-old chicks provides protection against infection within hours by intestinal colonisation–inhibition. However, the extent to which the oral application of live Salmonella wild-type or vaccine strains may induce an early invasion–inhibition effect is unknown. Potentially protective pre-treatment strains of Salmonella Enteritidis and Infantis were examined for their ability (i) to colonise the caeca, to invade the liver, to induce an influx of granulocytes in caecal mucosa and, (ii) for their capacity to inhibit the systemic invasion of homologous and heterologous Salmonella challenge organisms. The highly invasive strain Salmonella Enteritidis induced a strong influx of heterophils in the caecal mucosa followed by a complete invasion–inhibition of both homologous and heterologous Salmonella challenge organisms administered 24 h later. Pre-treatment with a less invasive Salmonella Infantis resulted in a lower influx of granulocytes in the caecal tissue followed by a complete invasion–inhibition of the homologous serovar Infantis but only an incomplete invasion–inhibition of heterologous serovars. This invasion–inhibition effect has not been described previously in chickens and should be considered in the development of novel live Salmonella vaccines to prevent an early invasion of extra-intestinal organs by Salmonella challenge organisms in young chicks.