Multi‐locus sequence typing of Salmonella enterica serovar Typhimurium isolates from wild birds in northern England suggests host‐adapted strain

Aims: Recent studies have suggested that Salmonella Typhimurium strains associated with mortality in UK garden birds are significantly different from strains that cause disease in humans and livestock and that wild bird strains may be host adapted. However, without further genomic characterization of these strains, it is not possible to determine whether they are host adapted. The aim of this study was to characterize a representative sample of Salm. Typhimurium strains detected in wild garden birds using multi‐locus sequence typing (MLST) to investigate evolutionary relationships between them. Methods and Results: Multi‐locus sequence typing was performed on nine Salm. Typhimurium strains isolated from wild garden birds. Two sequence types were identified, the most common of which was ST568. Examination of the public Salmonella enterica MLST database revealed that only three other ST568 isolates had been cultured from a human in Scotland. Two further isolates of Salm. Typhimurium were determined to be ST19. Conclusions: Results of MLST analysis suggest that there is a predominant strain of Salm. Typhimurium circulating among garden bird populations in the United Kingdom, which is rarely detected in other species, supporting the hypothesis that this strain is host adapted. Significance and Impact of the Study: Host–pathogen evolution is often assumed to lead to pathogens becoming less virulent to avoid the death of their host; however, infection with ST568 led to high mortality rates among the wild birds examined, which were all found dead at wild bird‐feeding stations. We hypothesize that by attracting unnaturally high densities of birds, wild bird‐feeding stations may facilitate the transmission of ST568 between wild birds, therefore reducing the evolutionary cost of this pathogen killing its host, resulting in a host‐adapted strain with increased virulence.     

Molecular typing of Salmonella enterica serovar Sofia in Australia by pulsed‐field gel electrophoresis and repetitive element PCR typing

Aims: In this study, we used two molecular fingerprinting methods to investigate the genetic and clonal relationship shared by Australian Salmonella Sofia isolates. Methods and Results: A total of 84 Australian Salm. Sofia isolates from various states in Australia were typed using pulsed‐field gel electrophoresis (PFGE) (XbaI and SpeI) and repetitive element PCR (REP1R‐I primer). The previous problem of DNA degradation of Salm. Sofia strains was solved by modifying the lysis solution used to treat the bacterial plugs, allowing Salm. Sofia to be subtyped using PFGE. Molecular typing of isolates resulted in the generation of eight XbaI, six SpeI and five REP1 pattern profiles. Individual typing methods showed low discrimination index values (<0·5), indicating the poor discriminatory ability of the methods. However, the combination of the typing methods was able to improve the discrimination of isolates, further dividing them into 16 subtypes and raising the index value to 0·721. Conclusions: The combination of typing methods was shown to be the best approach to fingerprint Salm. Sofia. The Australian Salm. Sofia isolates only showed limited genetic diversity and probably share a clonal relationship. A majority of the Salm. Sofia isolates were not geographically restricted with the predominant pattern subtype observed amongst the isolates from various states. Significance and Impact of the Study: We have successfully devised a PFGE protocol that counteracts DNase activity of Salm. Sofia, enabling typing of this serovar.     

Untreated and enzyme‐modified bovine whey products reduce association of Salmonella Typhimurium,Escherichia coli O157:H7 and Cronobacter malonaticus (formerly Enterobacter sakazakii) to CaCo‐2 cells

Aims: Adhesion of a micro‐organism to a cell surface is often considered to be the first step in pathogenesis. Inhibiting this process may have therapeutic effects in vivo. This study investigates the inhibitory effects of various bovine whey products on the association of Salm. Typhimurium, E. coli O157:H7 and C. malonaticus (formerly Enterobacter sakazakii) to the human CaCo‐2 cell line. Invasion of CaCo‐2 cells by Salm. Typhimurium and C. malonaticus was also examined. Methods and Results: Infection assays were performed by incubating pathogenic acteria with CaCo‐2 cells in the presence of untreated (UT) or enzyme‐modified (EM) whey products. Associated micro‐organisms were directly quantified by plate counts. Invasion of CaCo‐2 cells by Salm. Typhimurium and C. malonaticus in the presence/absence of test materials was also quantified using gentamicin protection assays. At a concentration of 40 mg ml^?1, some UT whey products reduced association and invasion, but this effect was enhanced following hydrolysis with porcine pancreatic lipase. Conclusions: Both UT and EM sweet whey protein concentrates (WPCs) were found to be particularly effective inhibitors of association and invasion. All EM whey products significantly (P < 0·05) inhibited invasion of C. malonaticus into epithelial cells, causing a 2‐log reduction in the quantity of these micro‐organisms internalized. Significance and Impact of the Study: The present study suggests that whey products can inhibit association to and invasion of CaCo‐2 cells by selected micro‐organisms and may be useful in the treatment and/or prevention of foodborne infections.     

Classification of Salmonella enterica serotypes from Australian poultry using repetitive sequence-based PCR

Aims: To evaluate a semi‐automated repetitive extragenic palindromic sequence‐based PCR (rep‐PCR) system for the classification of Salmonella serotypes from Australian poultry. Methods and Results: Using a DNA fingerprint library within the DiversiLab^® System, four separate databases were constructed (serogroup B, C, E and Other). These databases contained 483 serologically confirmed (reference laboratory) Salmonella isolates. A blinded set of Salmonella cultures (n = 155) were typed by rep‐PCR, matched against the internal library and compared with traditional serotyping. The predicted (Kullback–Leibler) serotype of 143 (92·3%) isolates matched traditional typing (P < 0·05). Of the 12 (7·7%) remaining isolates, ten (6·5%) resulted in ‘No Match’, one (0·65%) was incorrectly matched to the library (Salm. subsp 1 ser 4,12:‐:‐), and the other (0·65%) was referenced as Salm. ser. Sofia, whereas rep‐PCR and in‐house serotyping concurred as Salmonella serovar Typhimurium. Financial analysis showed higher material cost (215%) and a lower labour component (47·5%) for rep‐PCR compared with serotyping. Conclusion: The DiversiLab^® System, with serogroup databases, was successfully implemented as an adjunct for reference serotyping of Salmonella enterica. Significance and Impact of the Study: The DiversiLab^® System platform is a cost‐effective and easy‐to‐use system, which can putatively determine Salmonella enterica serotypes within a few hours.     

Kinetic model‐based prediction of the persistence of Salmonella enterica serovar Typhimurium under tropical agricultural field conditions

Aim: Present a kinetic model‐based approach for using isothermal data to predict the survival of manure‐borne enteric bacteria under dynamic conditions in an agricultural environment. Methods and Results: A model to predict the survival of Salmonella enterica serovar Typhimurium under dynamic temperature conditions in soil in the field was developed. The working hypothesis was that the inactivation phenomena associated with the survival kinetics of an organism in an agricultural matrix under dynamic temperature conditions is for a large part due to the cumulative effect of inactivation at various temperatures within the continuum registered in the matrix in the field. The modelling approach followed included (i) the recording of the temperature profile that the organism experiences in the field matrix, (ii) modelling the survival kinetics under isothermal conditions at a range of temperatures that were registered in the matrix in the field; and (iii) using the isothermal‐based kinetic models to develop models for predicting survival under dynamic conditions. The time needed for 7 log CFU g^?1Salmonella Typhimurium in manure and manure‐amended soil to reach the detection limit of the enumeration method (2 log CFU g^?1) under tropical conditions in the Central Agro‐Ecological Zone of Uganda was predicted to be 61–68 days and corresponded with observed CFU of about 2·2–3·0 log CFU g^?1, respectively. The Bias and Accuracy factor of the prediction was 0·71–0·84 and 1·2–1·4, respectively. Conclusions: Survival of Salm. Typhimurium under dynamic field conditions could be for 71–84% determined by the developed modelling approach, hence substantiating the working hypothesis. Significance and Impact of the Study: Survival kinetic models obtained under isothermal conditions can be used to develop models for predicting the persistence of manure‐borne enteric bacteria under dynamic field conditions in an agricultural environment.     

Inhibition of the type III secretion system by syringaldehyde protects mice from Salmonella enterica serovar Typhimurium

The invasiveness of Salmonella enterica serovar Typhimurium (S. Typhimurium) is closely associated with the Salmonella pathogenicity island (SPI)‐encoded type Ⅲ secretion system (T3SS), which can directly inject a series of effector proteins into eukaryotic cells to enable bacterial infection. In this study, syringaldehyde was identified as an effective inhibitor of the S. Typhimurium T3SS using an effector protein‐lactamase fusion reporter system. Syringaldehyde treatment could inhibit the expression of important effector proteins (SipA, SipB and SipC) at a concentration of 0.18 mM without affecting bacterial growth. Additionally, significant inhibition of bacterial invasion and cellular injury was observed following the syringaldehyde treatment in the co‐infection system of HeLa cells and S. Typhimurium. Furthermore, treatment with syringaldehyde provided systemic protection to mice infected with S. Typhimurium, reducing mortality (40.00%) and bacterial loads and relieving caecal damage and systemic inflammation. The results presented in this study indicate that syringaldehyde significantly affects T3SS activity and is a potential leading compound for treating S. Typhimurium infections.     

In vitro effect of subinhibitory concentrations of antibiotics on biofilm formation by clinical strains of Salmonella enterica serovar Typhimurium isolated in Slovakia

Aims: In this study, we examined the biofilm formation of 75 Salmonella enterica serovar Typhimurium (Salm. Typhimurium) human clinical isolates and the effect of subinhibitory concentrations (sub-MICs) of gentamicin, ciprofloxacin and cefotaxime on biofilm formation and exopolysaccharides (EPS) production. Methods and Results: Quantification of biofilm formation and EPS production were carried out using a modified microtitre plate assay and spectrophotometric method, respectively. The results indicate that 38 isolates (50·7%), which are predominantly of DT104 phage type, presented as the strong biofilm producers in vitro on plastic surface. When strains with the highest biofilm-forming capacity were grown in the presence of sub-MICs of gentamicin and ciprofloxacin, the inhibition of biofilm formation and EPS production was observed. In contrast, cefotaxime at 1/2 MIC (0·039 μg ml⁻¹) was able to significantly induce the production of biofilm as well as EPS in three isolates with nontypable and DT104 phage type, respectively. Conclusions: These results clearly indicate that all the three antibiotics tested are able to interfere with biofilm formation and EPS production by Salm. Typhimurium isolates. Significance and Impact of the Study: The current study demonstrated that cefotaxime at sub-MIC can be beneficial for the behaviour of pathogen Salm. Typhimurium in vitro.     

A synthetic furanone potentiates the effect of disinfectants on Salmonella in biofilm

Aims: To study a possible effect of a synthetic brominated furanone on biofilm formation and biofilm resistance to disinfectants in Salmonella enterica. Methods and Results: The effect of a synthetic furanone on biofilm formation of Salm. enterica serovar Agona and Salm. enterica serovar Typhimurium (11 strains of different origins) was evaluated in a microtiterplate assay. A significant reduction in biofilm build‐up in microtiterplates by the furanone was observed for seven of the strains tested. Biofilms by two Salm. Agona feed factory strains and the effects on survival after exposures to disinfectants (hypochlorite and benzalkonium chloride) were assessed for both strains. Pretreatment with furanone significantly potentiated the effect of the two disinfectants for both strains. Conclusions: The effect of disinfectants on Salmonella in biofilm was significantly enhanced when the biofilm was grown in the presence of furanone. This was probably because of an effect on biofilm architecture, composition and in some cases also biofilm build‐up. Significance and Impact of the Study: The present study gives valuable new knowledge in the fight against Salmonella biofilm in the environment because of the potentiated effect of conventional disinfectants.     

Antimicrobial activity of lemongrass oil against Salmonella enterica on organic leafy greens

Aims: We investigated the antimicrobial effectiveness of lemongrass essential oil on organic leafy greens, romaine and iceberg lettuces and mature and baby spinach, inoculated with Salmonella Newport. The influences of exposure times and abuse temperatures on bacterial survival were also investigated. Methods and Results: Leaf samples were washed, inoculated with Salm. Newport (6‐log CFU ml^?1) and dried. Inoculated leaves were immersed in solutions containing 0·1, 0·3 or 0·5% lemongrass oil in phosphate‐buffered saline for 1 or 2 min and then individually incubated at 4 or 8°C. Samples were taken at day 0, 1 and 3 for the enumeration of survivors. Compared to the PBS control, romaine and iceberg lettuces, and mature and baby spinach samples showed between 0·6–1·5‐log, 0·5–4·3‐log, 0·5–2·5‐log and 0·5–2·2‐log CFU g^?1 reductions in Salm. Newport by day 3, respectively. Conclusions: The antimicrobial activity of lemongrass oil against Salm. Newport was concentration and time dependent. The antimicrobial activity increased with exposure time; iceberg samples treated for 2 min generally showed greater reductions (P < 0·05) than those treated for 1 min (c. 1‐log reduction difference for 0·3 and 0·5% treatments). Few samples showed a difference between refrigeration and abuse temperatures. Significance and Impact of the Study: This study demonstrates the potential of lemongrass oil solutions to inactivate Salm. Newport on organic leafy greens.     

Antimicrobial potential of polyphenols extracted from almond skins

Aims: To evaluate the antimicrobial properties of flavonoid‐rich fractions derived from natural and blanched almond skins, the latter being a by‐product from the almond processing industry. Methods and Results: Almond skin extracts were tested against Gram‐negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica, Serratia marcescens), Gram‐positive bacteria (Listeria monocytogenes, Enterococcus hirae, Staphylococcus aureus, Enterococcus durans) and the yeast Candida albicans. Almond skin fractions were found to have antimicrobial activity against L. monocytogenes and Staph. aureus in the range 250–500 μg ml^?1, natural skins showing antimicrobial potential against the Gram‐negative Salm. enterica. The interactions between three almond skin flavonoids were also evaluated with isobolograms. Conclusions: Pairwise combinations of protocatechuic acid, naringenin and epicatechin showed both synergistic and indifferent interactions against Salm. enterica and Staph. aureus. Antagonism was observed against L. monocytogenes with all combinations tested. Further studies need to be performed to understand the mechanisms responsible for these interactions. Significance and Impact of the Study: Almond skins are a potential source of natural antimicrobials.     

Viability of zoonotic pathogens Escherichia coli and Salmonella in swine manure slurries with and without a urease inhibitor and thymol

Aims: To determine the effects of urine, a urease inhibitor and/or an odour‐reducing antimicrobial compound, on zoonotic pathogens in swine manure slurries. Methods and Results: Swine faeces were collected and blended with different amounts of urine. Marker strains of Salmonella enterica subsp. enterica serovar Typhimurium and Escherichia coli O157:H7 were inoculated into the manure slurries with or without the urease inhibitor, N‐(n‐butyl) thiophosphoric triamide, or the antimicrobial compound thymol. In slurries containing approximately 1 : 1 or 2 : 1 of urine and faeces, the decreases in colony forming units (CFU) of Salm. Typhimurium and E. coli were similar and few counts were observed after 14 days. When the urine content of the slurry was increased to 5 : 1, both strains died off rapidly. When NBPT was added to the slurries, pathogen CFU was not affected in the 1 : 1 slurry. The 2 : 1 and 5 : 1 slurries had higher urea content and NBPT increased CFU of both pathogens. Thymol addition rapidly killed the pathogens and few CFU of Salmonella or E. coli were observed after day 1. Conclusions: These experiments demonstrate that urea hydrolysis in swine manure affects pathogens. Inhibition of urea hydrolysis may promote pathogen viability. Significance and Impact of the Study: Manure amendments that prevent urea hydrolysis may promote pathogen persistence. Additional treatments with antimicrobials may be required to kill pathogens.     

Cross‐protective effect of acid‐adapted Salmonella enterica on resistance to lethal acid and cold stress conditions

Aims: To evaluate the cross‐protected Salmonella enterica cells under acid and cold stress conditions. Methods and Results: The acid‐adapted S. enterica cells were exposed to pH 4·0 at 4 and 20°C. Recovery of sublethally injured cells was estimated by the difference between the counts obtained on trypticase soy agar (TSA) and xylose lysine desoxycholate (XLD) agar. The survival curves of nonadapted and acid‐adapted S. enterica cells at pH 4·0 were fitted with Weibull distribution model. The recovery behaviour of injured S. enterica cells was estimated by the modified Gompertz parameters. Acid‐adapted S. enterica were more resistant to subsequent acid shock than the nonadapted cells. The numbers of nonadapted S. enterica cells were decreased by 4·57 and 7·55 log CFU ml^?1 at 4 and 20°C after 12‐day acid challenge, respectively. The acid adaptation induced cross‐protection and viable nonculturable (VBNC) state against low acid and cold stresses. The 7‐h adaptation showed the least recovery of injured cells. Conclusion: The results suggest that acid‐adapted S. enterica cells induced acid tolerance response and VBNC state. Significance and Impact of the Study: These results provide useful information for understanding the induction of cross‐protected and VBNC pathogens under various stresses, which might be needed in designing new food preservation strategies.     

Detection of Salmonella enterica in food using two-step enrichment and real-time polymerase chain reaction

Aims: A new real‐time polymerase chain reaction‐based method was developed for the detection of Salmonella enterica in food. Methods and Results: The method consisted of a novel two‐step enrichment involving overnight incubation in buffered peptone water and a 5‐h subculture in Rappaport–Vassiliadis medium, lysis of bacterial cells and a Salmonella‐specific 5′‐nuclease real‐time PCR with an exogenous internal amplification control. Because a two‐step enrichment was used, the detection limit for dead S. enterica cells in artificially contaminated ice cream and salami samples was high at 10⁷ CFU (25 g)^?1, eliminating potential false‐positive results. When the method was evaluated with a range of 100 naturally contaminated food samples, three positive samples were detected by both the real‐time PCR‐based method and by the standard microbiological method, according to EN ISO 6579. When the real‐time PCR‐based method was evaluated alongside the standard microbiological method according to EN ISO 6579 with 36 food samples artificially contaminated at a level of 10⁰ CFU (25 g)^?1, identical results were obtained from both methods. Conclusions: The real‐time PCR‐based method involving a two‐step enrichment produced equivalent results to EN ISO 6579 on the day after sample receipt. Significance and Impact of the Study: The developed method is suitable for rapid detection of S. enterica in food.     

Association with pathogenic bacteria affects life‐history traits and population growth in Caenorhabditis elegans

Determining the relationship between individual life‐history traits and population dynamics is an essential step to understand and predict natural selection. Model organisms that can be conveniently studied experimentally at both levels are invaluable to test the rich body of theoretical literature in this area. The nematode Caenorhabditis elegans, despite being a well‐established workhorse in genetics, has only recently received attention from ecologists and evolutionary biologists, especially with respect to its association with pathogenic bacteria. In order to start filling the gap between the two areas, we conducted a series of experiments aiming at measuring life‐history traits as well as population growth of C. elegans in response to three different bacterial strains: Escherichia coli OP50, Salmonella enterica Typhimurium, and Pseudomonas aeruginosa PAO1. Whereas previous studies had established that the latter two reduced the survival of nematodes feeding on them compared to E. coli OP50, we report for the first time an enhancement in reproductive success and population growth for worms feeding on S. enterica Typhimurium. Furthermore, we used an age‐specific population dynamic model, parameterized using individual life‐history assays, to successfully predict the growth of populations over three generations. This study paves the way for more detailed and quantitative experimental investigation of the ecology and evolution of C. elegans and the bacteria it interacts with, which could improve our understanding of the fate of opportunistic pathogens in the environment.     

Growth potential of faecal bacteria in simulated psychrophilic/mesophilic zones during composting of organic waste

Aim: This study investigated the growth potential of Salmonella serotype Typhimurium and faecal indicator organisms in compost materials and the correlation between bacterial growth potential and the physico‐chemical composition of the compost substrate and temperature. Methods and Results: Survival of Salm. Typhimurium, Enterococcus spp. and total coliforms at 14, 24 and 37°C was determined in material of different degrees of maturity collected from composting plants for household waste and manure. All three micro‐organisms showed the potential for growth in the material from active composts (Solvita index 4) but inactivation generally occurred over time in mature compost material (Solvita index 7–8). Conclusions: Salm. Typhimurium had the potential for growth in psychrophilic/mesophilic (P/M) zones of immature compost material and its growth potential correlated negatively with the maturity of the compost and the temperature within the simulated P/M zone. Significance and Impact of the Study: The risk of pathogen regrowth in P/M zones during organic waste composting further emphasizes the importance of good management practices and of avoiding P/M zones in combination with low compost maturity.     

ChIP‐seq and transcriptome analysis of the OmpR regulon of Salmonella enterica serovars Typhi and Typhimurium reveals accessory genes implicated in host colonization

OmpR is a multifunctional DNA binding regulator with orthologues in many enteric bacteria that exhibits classical regulator activity as well as nucleoid‐associated protein‐like characteristics. In the enteric pathogen Salmonella enterica, using chromatin immunoprecipitation of OmpR:FLAG and nucleotide sequencing, 43 putative OmpR binding sites were identified in S. enterica serovar Typhi, 22 of which were associated with OmpR‐regulated genes. Mutation of a sequence motif (TGTWACAW) that was associated with the putative OmpR binding sites abrogated binding of OmpR:6×His to the tviA upstream region. A core set of 31 orthologous genes were found to exhibit OmpR‐dependent expression in both S. Typhi and S. Typhimurium. S. Typhimurium‐encoded orthologues of two divergently transcribed OmpR‐regulated operons (SL1068–71 and SL1066–67) had a putative OmpR binding site in the inter‐operon region in S. Typhi, and were characterized using in vitro and in vivo assays. These operons are widely distributed within S. enterica but absent from the closely related Escherichia coli. SL1066 and SL1067 were required for growth on N‐acetylmuramic acid as a sole carbon source. SL1068–71 exhibited sequence similarity to sialic acid uptake systems and contributed to colonization of the ileum and caecum in the streptomycin‐pretreated mouse model of colitis.     

Spontaneous and transient defence against bacteriophage by phase‐variable glucosylation of O‐antigen in Salmonella enterica serovar Typhimurium

As natural killers of bacteria, bacteriophages have forced bacteria to develop a variety of defence mechanisms. The alteration of host receptors is one of the most common bacterial defence strategies against phage infection, which completely blocks phage attachment but comes at a potential fitness cost to the bacteria. Here, we report the cost‐free, transient emergence of phage resistance in Salmonella enterica subspecies enterica serovar Typhimurium through a phase‐variable modification of the O‐antigen. Phage SPC35 typically requires BtuB as a host receptor but also uses the Salmonella O12‐antigen as an adsorption‐assisting apparatus for the successful infection of S. Typhimurium. The α‐1,4‐glucosylation of galactose residues in the O12‐antigen by phase variably expressed O‐antigen glucosylating genes, designated the ^{LT 2} gtrABC1 cluster, blocks the adsorption‐assisting function of the O12‐antigen. Consequently, it confers transient SPC35 resistance to Salmonella without any mutations to the btuB gene. This temporal switch‐off of phage adsorption through phase‐variable antigenic modification may be widespread among Gram‐negative bacteria‐phage systems.     

Probiotic‐mediated competition,exclusion and displacement in biofilm formation by food‐borne pathogens

The objective of this study was to examine the inhibitory effect of probiotic strains on pathogenic biofilm formation in terms of competition, exclusion and displacement. Probiotic strains (Lactobacillus acidophilus KACC 12419, Lact. casei KACC 12413, Lact. paracasei KACC 12427 and Lact. rhamnosus KACC 11953) and pathogens (Salmonella Typhimurium KCCM 40253 and Listeria monocytogenes KACC 12671) were used to evaluate the auto‐aggregation, hydrophobicity and biofilm formation inhibition. The highest auto‐aggregation abilities were observed in Lact. rhamnosus (17·5%), Lact. casei (17·2%) and Lact. acidophilus (15·1%). Salm. Typhimurium had the highest affinity to xylene, showing the hydrophobicity of 53·7%. The numbers of L. monocytogenes biofilm cells during the competition, exclusion and displacement assays were effectively reduced by more than 3 log when co‐cultured with Lact. paracasei and Lact. rhamnosus. The results suggest that probiotic strains can be used as alternative way to effectively reduce the biofilm formation in pathogenic bacteria through competition, exclusion and displacement.

Significance and Impact of the Study

This study provides new insight into biofilm control strategy based on probiotic approach. Probiotic strains effectively inhibited the biofilm formation of Listeria monocytogenes through the mechanisms of competition, exclusion and displacement. These findings contribute to better understand the probiotic‐mediated competition, exclusion and displacement in biofilm formation by pathogens.     

Survival of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium in manure and manure‐amended soil under tropical climatic conditions in Sub‐Saharan Africa

Aims: To establish the fate of Escherichia coli O157:H7 and Salmonella Typhimurium in manure and manure‐amended agricultural soils under tropical conditions in Sub‐Saharan Africa. Methods and Results: Survival of nonvirulent E. coli O157:H7 and Salm. Typhimurium at 4 and 7 log CFU g^?1 in manure and manure‐amended soil maintained at ≥80% r.h. or exposed to exclusive field or screen house conditions was determined in the Central Agro‐Ecological Zone of Uganda. Maintaining the matrices at high moisture level promoted the persistence of high‐density inocula and enhanced the decline of low‐density inocula in the screen house, but moisture condition did not affect survival in the field. The large majority of the survival kinetics displayed complex patterns corresponding to the Double Weibull model. The two enteric bacteria survived longer in manure‐amended soil than in manure. The 7 log CFU g^?1E. coli O157:H7 and Salm. Typhimurium survived for 49–84 and 63–98 days, while at 4 log CFU g^?1, persistence was 21–28 and 35–42 days, respectively. Conclusions: Under tropical conditions, E. coli O157:H7 and Salm. Typhimurium persisted for 4 and 6 weeks at low inoculum density and for 12 and 14 weeks at high inoculum density, respectively. Significance and Impact of the Study: Persistence in the tropics was (i) mostly shorter than previously observed in temperate regions thus suggesting that biophysical conditions in the tropics might be more detrimental to enteric bacteria than in temperate environments; (ii) inconsistent with published data isothermally determined previously hence indicating the irrelevance of single point isothermal data to estimate survival under dynamic temperature conditions.