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Concept: XLD agar


ABSTRACT We investigated the application capabilities of a laser optical sensor, BARDOT (bacterial rapid detection using optical scatter technology) to generate differentiating scatter patterns for the 20 most frequently reported serovars of Salmonella enterica. Initially, the study tested the classification ability of BARDOT by using six Salmonella serovars grown on brain heart infusion, brilliant green, xylose lysine deoxycholate, and xylose lysine tergitol 4 (XLT4) agar plates. Highly accurate discrimination (95.9%) was obtained by using scatter signatures collected from colonies grown on XLT4. Further verification used a total of 36 serovars (the top 20 plus 16) comprising 123 strains with classification precision levels of 88 to 100%. The similarities between the optical phenotypes of strains analyzed by BARDOT were in general agreement with the genotypes analyzed by pulsed-field gel electrophoresis (PFGE). BARDOT was evaluated for the real-time detection and identification of Salmonella colonies grown from inoculated (1.2 × 10(2) CFU/30 g) peanut butter, chicken breast, and spinach or from naturally contaminated meat. After a sequential enrichment in buffered peptone water and modified Rappaport Vassiliadis broth for 4 h each, followed by growth on XLT4 (~16 h), BARDOT detected S. Typhimurium with 84% accuracy in 24 h, returning results comparable to those of the USDA Food Safety and Inspection Service method, which requires ~72 h. BARDOT also detected Salmonella (90 to 100% accuracy) in the presence of background microbiota from naturally contaminated meat, verified by 16S rRNA sequencing and PFGE. Prolonged residence (28 days) of Salmonella in peanut butter did not affect the bacterial ability to form colonies with consistent optical phenotypes. This study shows BARDOT’s potential for nondestructive and high-throughput detection of Salmonella in food samples. IMPORTANCE High-throughput screening of food products for pathogens would have a significant impact on the reduction of food-borne hazards. A laser optical sensor was developed to screen pathogen colonies on an agar plate instantly without damaging the colonies; this method aids in early pathogen detection by the classical microbiological culture-based method. Here we demonstrate that this sensor was able to detect the 36 Salmonella serovars tested, including the top 20 serovars, and to identify isolates of the top 8 Salmonella serovars. Furthermore, it can detect Salmonella in food samples in the presence of background microbiota in 24 h, whereas the standard USDA Food Safety and Inspection Service method requires about 72 h.

Concepts: Growth medium, Food, Pathogen, Agar plate, Bacteria, Microbiology, XLD agar, Salmonella enterica


Microbiologists have been using agar growth medium for over 120 years. It revolutionized microbiology in the 1890’s when microbiologists were seeking effective methods to isolate microorganisms, which led to the successful cultivation of microorganisms as single clones. But there has been a disparity between total cell counts and cultivable cell counts on plates, often referred to as the “great plate count anomaly”, that has long been a phenomenon that still remains unsolved. Here we report that a common practice microbiologists have been employing to prepare agar medium has a hidden pitfall: when phosphate was autoclaved together with agar to prepare solid growth media (PT medium), total colony counts were remarkably lower when compared with those grown on agar plates in which phosphate and agar were separately autoclaved and mixed right before solidification (PS medium). We used a pure culture Gemmatimonas aurantiaca T-27(T) and three representative sources of environmental samples, soil, sediment and water, as inocula and compared colony counts between PT and PS agar plates. There were higher numbers of colony forming units (CFUs) on PS medium compared to PT medium using G. aurantiaca or any of the environmental samples. Chemical analysis of PT agar plates suggested that hydrogen peroxide was contributing to growth inhibition. Comparison of 454 pyrosequences of the environmental samples to the isolates revealed that taxa grown on PS medium were more reflective of the original community structure than those grown on PT medium. Moreover, more hitherto-uncultivated microbes grew on PS than on PT medium.

Concepts: Metabolism, Petri dish, XLD agar, Agar, Growth medium, Bacteria, Microbiology, Agar plate


Different studies have reported the prevalence of Salmonella in turtles and its role in reptile-associated salmonellosis in humans, but there is a lack of scientific literature related with the epidemiology of Campylobacter in turtles. The aim of this study was to evaluate the prevalence of Campylobacter and Salmonella in free-living native (Emys orbicularis, n=83) and exotic (Trachemysscripta elegans, n=117) turtles from 11 natural ponds in Eastern Spain. In addition, different types of samples (cloacal swabs, intestinal content and water from Turtle containers) were compared. Regardless of the turtle species, natural ponds where individuals were captured and the type of sample taken, Campylobacter was not detected. Salmonella was isolated in similar proportions in native (8.0±3.1%) and exotic (15.0±3.3%) turtles (p=0.189). The prevalence of Salmonella positive turtles was associated with the natural ponds where animals were captured. Captured turtles from 8 of the 11 natural ponds were positive, ranged between 3.0±3.1% and 60.0±11.0%. Serotyping revealed 8 different serovars among four Salmonella enterica subspecies: S. enterica subsp. enterica (n = 21), S. enterica subsp. salamae (n = 2), S. enterica subsp. diarizonae (n = 3), and S. enterica subsp. houtenae (n = 1). Two serovars were predominant: S. Thompson (n=16) and S. typhimurium (n=3). In addition, there was an effect of sample type on Salmonella detection. The highest isolation of Salmonella was obtained from intestinal content samples (12.0±3.0%), while lower percentages were found for water from the containers and cloacal swabs (8.0±2.5% and 3.0±1.5%, respectively). Our results imply that free-living turtles are a risk factor for Salmonella transmission, but do not seem to be a reservoir for Campylobacter. We therefore rule out turtles as a risk factor for human campylobacteriosis. Nevertheless, further studies should be undertaken in other countries to confirm these results.

Concepts: Species, XLD agar, Epidemiology, 1984 Rajneeshee bioterror attack, Microbiology, Turtle, Salmonella, Salmonella enterica


Exosomes, molecular cargos secreted by almost all mammalian cells, are considered as promising biomarkers to identify many diseases including cancers. However, the small size of exosomes (30-200 nm) poses serious challenges on their isolation from the complex media containing a variety of extracellular vesicles (EVs) of different sizes, especially in small sample volumes. Here we present a viscoelasticity-based microfluidic system to directly separate exosomes from cell culture media or serum in a continuous, size-dependent, and label-free manner. Using a small amount of biocompatible polymer as the additive into the media to control the viscoelastic forces exerted on EVs, we are able to achieve a high separation purity (> 90 %) and recovery (> 80 %) of exosomes. The proposed technique may serve as a versatile platform to facilitate exosome analyses in diverse biochemical applications.

Concepts: Microbiology, DNA, Cell biology, Bacteria, XLD agar, Cell, Cell culture, Growth medium


Objective: To explore the effects of combined application of culture supernatant of human umbilical cord mesenchymal stem cells (hUCMSCs) and ciprofloxacin on Staphylococcus aureus (SA) in vitro. Methods: hUCMSCs were isolated from umbilical cord tissue of full-term healthy fetus after cesarean section and cultured. Cells in the third passage were used in the experiments after identification. SA strains isolated from wounds of burn patients in our burn wards were used in the experiments. Cells were divided into 0, 10, 100, and 1 000 ng/mL lipopolysaccharide (LPS) groups according to the random number table (the same dividing method below). Cells were cultured with culture medium of mesenchymal stem cells (MSCs) after being treated with medium containing the corresponding mass concentrations of LPS for 12 h. At post culture hour (PCH) 6, 12, and 24, 6 wells of culture supernatant of cells in each group were obtained to measure the content of LL-37 with enzyme-linked immunosorbent assay. Ninety blood agar plates were divided into ciprofloxacin control group (CC), ciprofloxacin+ supernatant group (CS), and ciprofloxacin+ supernatant+ LL-37 antibody group (CSL), with 30 blood agar plates in each group. Blood agar plates in group CC were coated with 1.5×10(8) colony forming unit (CFU)/mL bacteria solution prepared with normal saline. Blood agar plates in group CS were coated with 1.5×10(8) CFU/mL bacteria solution prepared with normal saline and culture supernatant of hUCMSCs (cultured by culture medium of MSCs, the same below) in double volume of normal saline. Blood agar plates in group CSL were coated with 1.5×10(8) CFU/mL bacteria solution prepared with normal saline, culture supernatant of hUCMSCs in double volume of normal saline, and 2.6 μL LL-37 antibody in the concentration of 2 μg/mL. At PCH 12, 24, and 48, 10 blood agar plates of each group were harvested to observe the distribution of SA colony on blood agar plate and to measure the diameter of bacterial inhibition ring of ciprofloxacin. The minimum inhibitory concentration (MIC) of ciprofloxacin against SA of each group was recorded. Fractional inhibitory concentration (FIC) indexes of ciprofloxacin in groups CS and CSL at PCH 12, 24, and 48 were calculated, and the effect of synergy was evaluated. Data were processed with analysis of variance of factorial design, one-way analysis of variance, LSD-t test, Kruskal-Wallis test, and Mann-Whitney U test. Results: (1) At each PCH, the content of LL-37 in culture supernatant of cells in 10, 100, and 1 000 ng/mL LPS groups was higher than that in 0 ng/mL LPS group (with t values from 11.22 to 33.36, P values below 0.01); the content of LL-37 in culture supernatant of cells in 100 and 1 000 ng/mL LPS groups was higher than that in 10 ng/mL LPS group (with t values from 2.24 to 18.73, P<0.05 or P<0.01); the content of LL-37 in culture supernatant of cells in 1 000 ng/mL LPS group was higher than that in 100 ng/mL LPS group (with t values from 12.46 to 14.70, P values below 0.01). (2) At PCH 12, 24, and 48, the bacterial colonies in groups CC, CS, and CSL began to integrate over time. At PCH 12, 24, and 48, the diameters of bacterial inhibition ring of ciprofloxacin in group CC were 26, 24, and 23 mm, respectively, with no obvious change. At PCH 12, 24, and 48, the diameters of bacterial inhibition ring of ciprofloxacin in groups CS and CSL were 82, 71, 68 mm, and 74, 59, 56 mm, respectively, significantly longer than those of group CC. (3) At each PCH, the MIC of ciprofloxacin against SA was significantly higher in group CC than in groups CS and CSL (with Z values from 6.22 to 6.71, P values below 0.01); the MIC of ciprofloxacin against SA was significantly higher in group CSL than in group CS (with Z values all equal to 6.72, P values below 0.01). (4) FIC indexes of ciprofloxacin in groups CS and CSL at PCH 12, 24, and 48 were 0.011, 0.032, 0.032, and 0.122, 0.350, 0.350, respectively. The results indicated that culture supernatant of hUCMSCs had synergistically antibacterial effect on ciprofloxacin. Conclusions: hUCMSCs can secrete LL-37, and the secretion level is increased with increase of LPS concentration. Combination of culture supernatant of hUCMSCs and ciprofloxacin can decrease the dosage of ciprofloxacin in resisting SA. Once LL-37 is neutralized, the synergistically antibacterial effect of culture supernatant of hUCMSCs is decreased.

Concepts: Mesenchymal stem cell, Pseudomonas aeruginosa, XLD agar, Bacteria, Bone marrow, Petri dish, Growth medium, Agar plate


In recent years, several pet food recalls have been attributed to Salmonella contamination. In addition to the negative impacts on animal health, Salmonella-contaminated pet foods have been linked to infection in humans. With that in mind, the U.S. Food and Drug Administration has set forth a zero-tolerance policy for Salmonella in pet foods. Typically, pet foods are extruded or processed at high temperatures that are sufficient to reduce pathogenic bacteria. However, the possibility for postextrusion contamination still exists. One potential method to reduce the risk of postextrusion contamination of pet foods with Salmonella is through the addition of a chemical additive coating. The objective of this research was to evaluate the ability of β-hydroxy-β-methylbutyrate (HMB), in either free acid (HMBFA) or calcium salt (CaHMB) form, to reduce postextrusion contamination of dry extruded dog kibble with Salmonella. Three trials were conducted with HMBFA and CaHMB coated onto the kibbles at levels of 0, 0.1, 0.3, 0.5, 0.9, and 1.5% (w/w). The coated kibbles were then inoculated with Salmonella enterica subsp. enterica Enteritidis (ATCC 13076), with enumeration done on days 0, 1, 2, 7, and 14 postinoculation. Subsamples on each day were serially diluted, spread plated to xylose lysine deoxycholate agar, and incubated at 37°C for 24 h. Salmonella colonies were then counted and log CFU per gram was calculated. The 1.5% HMBFA reduced counts by 4.9 ± 0.2 log units on day 1, whereas the positive control only decreased 2.2 ± 0.1 log units (P < 0.0001). The 1.5% CaHMB level decreased counts by 7.1 ± 0.04 log units by day 7 compared with the control decrease of 2.1 ± 0.1 log units (P < 0.0001). All HMBFA and CaHMB treatments resulted in the elimination of detectable Salmonella counts by day 14 (P < 0.0001 versus controls). In conclusion, HMB coating was effective at reducing Salmonella artificially inoculated to dog kibbles in a model of postextrusion contamination.

Concepts: Pathogenic bacteria, Pathogen, Bacteria, Pet foods, Microbiology, 2007 pet food recalls, XLD agar, Salmonella enterica


The current study was conducted to evaluate the ability to recover Salmonella from shell egg contents by culture methods. A total of 4,000 eggs were obtained from a grading and packing center located in the Gyeonggi Province of South Korea, and 200 samples were created by pooling 20 broken eggs. The pooled samples were held at room temperature for 4 d before a 25-mL aliquot of each pool was added to 225 mL of modified trypticase soy broth (mTSB) and incubated at 35°C for 24 ± 2 h. A loopful of the culture was streaked onto chromogenic Druggan-Forsythe-Iversen (DFI) agar and incubated at 36 ± 1°C for 18-24 h. In addition, 1 mL and/or 0.1 mL of the mTSB cultures were added to 10 mL of Muller-Kauffmann tetrathionate with novobiocin (MKTTn) or Rappaport-Vassiliadis (RV) broth, and they were incubated for 24 ± 2 h at 35 ± 2°C or 42 ± 0.2°C, respectively. A loopful from these cultures was streaked onto Brilliant Green (BG), xylose lysine deoxycholate (XLD), and bismuth sulfite (BS) agar plates, respectively. Directly streaking onto DFI agar revealed the presence of Salmonella in 14 out of the 200 pooled samples (7%); whereas the combination of RV medium and BG, XLD, and BS agar detected the pathogen in only 9 (4.5%), 7 (3.5%), and 3 (1.5%) of the pooled samples, respectively. When MKTTn broth was used, Salmonella was detected in 7 (3.5%), 2 (1%), and 0 (0%) of the samples when streaked onto BG, XLD, and BS agar, respectively. The results indicate that direct plating onto DFI agar without enrichment was the most suitable among the methods evaluated in this study for detecting Salmonella in raw shell egg contents with a low microbial load.

Concepts: Swimming pool, Egg yolk, Essential amino acid, Growth medium, The Culture, Egg white, Agar plate, XLD agar


Detection of Salmonella in some spices, such as cloves, remains a challenge due to their inherent antimicrobial properties. The purpose of this study was to develop an effective detection method for Salmonella from spices using cloves as a model. Two clove varieties, Ceylon and Madagascar, were used in the study. Cloves were inoculated with Salmonella enterica subsp. enterica serotypes Montevideo, Typhimurium, or Weltevreden at about 1, 3, or 6 log CFU/25 g. Two test portion sizes, 10 and 25 g, were compared. After adding Trypticase soy broth (TSB) to the weighed cloves for preenrichment, three preenrichment methods were compared: cloves were left in the TSB for 24 h during preenrichment (PreE1), or the cloves-TSB mixture was shaken vigorously for 30 s (PreE2) or 60 s (PreE3), and the decanted material was transferred to a new bag for 24 h of preenrichment. The rest of the procedures were carried out according to the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM). At the low inoculation level (<1 log CFU/25 g), the detection rate was low across the three preenrichment methods, with the highest for PreE3 and lowest for PreE1. At the medium and high inoculation levels (3 and 6 log CFU/25 g), all samples from PreE2 and PreE3 were positive for Salmonella , whereas PreE1 produced only 12 positive samples from the 48 samples at the medium inoculation level and 38 positive samples from the 48 samples at the high inoculation level. Therefore, PreE3 with 25 g of cloves per sample was more effective than the other two tested methods. This newly designed method was then validated by comparing with the BAM method in six trials, with each trial consisting of 40 test samples. The results showed that PreE3 detected Salmonella from 88 of 120 inoculated test samples compared with only 31 positive from 120 test samples with the BAM method. Thus, our newly designed method PreE3 was more sensitive and easier to operate than the current BAM method for detection of Salmonella in cloves.

Concepts: Antimicrobial, Inoculation, The Medium, Harshad number, XLD agar, Clove, Spice, Salmonella enterica


Culture-based detection of nontyphoidal Salmonella spp. in foods requires at least four working days; therefore, new detection methods that shorten the test time are needed. In this study, we developed a novel single-step Salmonella enrichment broth, SSE-1, and compared its detection capability with that of commercial single-step ONE broth-Salmonella (OBS) medium and a conventional two-step enrichment method using buffered peptone water and Rappaport-Vassiliadis soy broth (BPW-RVS). Minimally processed lettuce samples were artificially inoculated with low levels of healthy and cold-injured Salmonella Enteritidis (10(0) or 10(1) colony-forming unit/25 g), incubated in OBS, BPW-RVS, and SSE-1 broths, and streaked on xylose lysine deoxycholate (XLD) agar. Salmonella recoverability was significantly higher in BPW-RVS (79.2%) and SSE-1 (83.3%) compared to OBS (39.3%) (p < 0.05). Our data suggest that the SSE-1 single-step enrichment broth could completely replace two-step enrichment with reduced enrichment time from 48 to 24 h, performing better than commercial single-step enrichment medium in the conventional nonchromogenic Salmonella detection, thus saving time, labor, and cost.

Concepts: Daylight saving time, Broth, Growth medium, Salmonella, Salmonellosis, Salmonella enterica, XLD agar


The objective of this study was to develop a predictive model for the inactivation of Salmonella spp. in ground beef jerky as a function of temperature (T), pH, potassium sorbate (PS), and final water activity (aw). Following a central composite design, ground beef was combined with PS (0 to 0.3%, w/w), pH adjusted from 5 to 7, inoculated with a cocktail of 6 serotypes of Salmonella spp. and heat processed at temperatures between 65 and 85°C until the final aw ranging from 0.65 to 0.85 was achieved. Surviving Salmonella cells were enumerated on tryptic soy agar overlaid with xylose lysine deoxycholate agar (pre-tempered to 47°C) after incubation for 48h at 30°C. Bacterial inactivation was quantified in terms of logarithmic reductions of Salmonella counts (log10CFU/g) and inactivation rate (log10(CFU/g)/h). The results indicated that pH, PS and T significantly (p<0.05) interacted to inactivate Salmonella in beef jerky. Decreasing meat pH significantly (p<0.05) increased the efficacy of PS and T to reduce the levels of Salmonella spp. Beef jerky processed at 82°C, pH5.5, with 0.25% PS to a final aw of 0.7 resulted in a maximum Salmonella logarithmic reduction of 5.0log10CFU/g and an inactivation rate of 1.3log10(CFU/g)/h. The predictive model developed can be used to effectively design drying processes for beef jerky under low humidity conditions and thereby, ensuring an adequate degree of protection against risks associated with Salmonella spp.

Concepts: Enzyme, Heat, Entropy, Trigraph, Thermodynamics, Biltong, Sodium, XLD agar