Effect of chlortetracycline on the spread of R-100 plasmid-containing Escherichia coli BEL15R from experimentally infected pigs to uninfected pigs and chicks

Swine from two herds with different histories of antibiotic exposure were fed diets containing 0 or 55 mg of chlortetracycline (CTC)/kg. One of five pigs in each herd-diet treatment group was infected orally with Escherichia coli strain BEL15R that was resistant to nalidixic acid (NA), chloramphenicol (C), streptomycin (S), sulfamethizole (TH) and tetracycline (TE). Effects of CTC on the quantity and duration of fecal shedding of E. coli BEL15R and on the transmission of strain BEL15R and its R-100 plasmid from infected pigs to uninfected pigs and chicks were determined. Quantity and duration of shedding were greater in infected antibiotic-herd pigs than in infected nonantibiotic-herd pigs. Feeding on CTC increased the duration of shedding in infected pigs from both herds. Strain BEL15R colonized and was shed in one uninfected antibiotic pig in each treatment group, but it did not colonize in any of the uninfected nonantibiotic-herd pigs or in the uninfected chicks. In vivo transfer of resistance to C, S, TH and TE occurred in the infected antibiotic-herd pigs but not in the infected nonantibiotic-herd pigs. Transfer of the R-100 plasmid occurred from the infected to the uninfected antibiotic-herd pigs and to the uninfected chicks housed near the antibiotic-herd pigs fed CTC, but not to the chicks housed with the antibiotic-herd pigs fed the control diet. No transfer of resistance occurred from the infected nonantibiotic-herd pigs fed either CTC or control diet.

Effect of Time and Method of Aging on the Composition of the Microflora of Beef Loins and Corresponding Steaks 1

Twenty-four cattle were slaughtered at a commercial packing plant. Carcasses were chilled for 72 h at 4°C, and then fabricated into wholesale cuts. Right loins were vacuum packaged and aged for 1, 3 and 5 weeks at 4°C while the left loins were aged conventionally for 1, 2 and 3 weeks at the same temperature. Core samples were removed from both ends of the conventionally and vacuum-aged loins and analyzed using standard microbiological procedures. Isolates from aerobic plates incubated at 35 and 20°C, and from gram-negative enteric plates were used to determine the composition of the aerobic (35 and 20°C) and gram-negative enteric microflora. Loins were cut into 2.54-cm thick steaks, wrapped in oxygen permeable fresh meat wrap and placed at 4°C in a simulated retail meat case. After 1, 3, 6 and 7 d, steaks were evaluated using the same microbiological techniques used for loins. Vacuum packaged loins had lower percentages of pseudomonads than conventionally aged loins after 1 and 3 w of aging. The lactobacilli were higher in vacuum packaged loins than in conventionally aged loins after 3 weeks. The aerobic (35 and 20°C) microflora of the retail cuts generally reflected the flora of the loins from which they were prepared. Enterobacteriaceae tended to be higher in vacuum packaged loins and corresponding steaks, and constituted 34% of the flora of the vacuum packaged loins after 5 weeks of aging.

Effect of Time and Method of Aging on the Microbiological Quality of Beef Loins and Corresponding Steaks 1

Twenty-four cattle were slaughtered at a commercial packing plant. Carcasses were chilled for 72 h at 4°C, and then fabricated into wholesale cuts. Right loins were vacuum packaged and aged for 1, 3 and 5 weeks at 4°C while the left loins were aged conventionally for 1, 2 and 3 weeks at the same temperature. Core samples were removed from both ends of the loins and were analyzed using standard microbiological procedures. Loins were cut into 2.54-cm thick steaks, wrapped in oxygen-permeable fresh meat wrap and placed at 4°C in a simulated meat case. On days 1, 3, 6 and 7 steaks were evaluated using the same microbial technique used for loins. Counts generally were similar for conventionally and vacuum-aged loins after aging 1 week. After 3 weeks of aging, the vacuum-aged loins had higher (P<.05) coliform, anaerobic, streptococcus and lactobacillus counts. In addition to these counts, the aerobic (20°C and 35°C) of vacuum-aged loins increased (P<.05) after an additional 2 weeks of aging. Counts of steaks from 1-week old loins generally were not different as a result of method of loin aging. Lactobacillus counts of steaks from 3-week old loins differed as a result of method of aging. Except for higher lactobacillus counts, counts of steaks from 5-week old vacuum aged loins were similar to the counts of steaks from loins which were vacuum-aged for 3 weeks.

Antibiotic resistance in anaerobic and coliform bacteria from the intestinal tract of swine fed therapeutic and subtherapeutic concentrations of chlortetracycline

Chlortetracycline (CTC) resistance in anaerobic and coliform bacteria was examined in the large intestines of two groups of growing pigs that had received antibiotic-free diets since weaning. One group of pigs was from a low resistance herd (LR) that had not received antibiotics for 8 yr, while the other group of pigs was from a high resistance herd (HR) that routinely received antibiotics. After a 20-d adjustment period in a common production facility, LR pigs on an antibiotic-free diet had lower proportions of anaerobes (27%) and coliforms (22%) that were resistant to 25 micrograms CTC/ml than did similarly fed HR pigs (81 and 48%, respectively). Continued maintenance of LR and HR pigs on the antibiotic-free diet in a common production facility tended to increase resistance in anaerobes and coliforms from LR pigs to levels comparable with those in HR pigs at the end of an 85-d feeding trial, but not after 14 d. Administration of CTC in the feed at therapeutic (220 micrograms/g for 14 d) and growth-promoting (27.5 micrograms/g for 85 d) levels markedly increased percentages of resistant anaerobes and coliforms in LR pigs, but not in HR pigs after 14 d. Similar antibiotic effects were not seen at the end of the 85-d trial. These data demonstrate that, while exposure to antibiotics in feed may increase resistance in intestinal populations, other factors such as environment, cross-contamination and herd history can also influence the observed level of antibiotic resistance in swine.

Some characteristics and antibiotic resistance of anaerobic bacteria from the ceca and colons of pigs fed chlortetracycline-containing and unmedicated diets

Morphology and end product patterns were used to group 112 bacterial isolates from the ceca and colons of pigs fed unmedicated and chlortetracycline-containing diets. Most of the isolates (68%) were resistant to chlortetracycline (greater than 4 micrograms/ml). Chlortetracycline resistance was associated with all groups of anaerobic and facultative bacteria.

Antibiotic resistance of fecal coliforms after long-term withdrawal of therapeutic and subtherapeutic antibiotic use in a swine herd

Tetracycline resistance of fecal coliforms isolated from swine decreased from 82 to 42%, a decrease of less than 50%, after the use of all forms of antimicrobial agents were discontinued in the herd for 126 months.

Identification of Staphylococcus species of bovine origin with the API Staph-Ident system

The API Staph-Ident system was evaluated as a means for identifying the species of bovine strains of staphylococci routinely isolated from quarter-milk samples. The species identity of 314 of 581 (54%) isolates of staphylococci was correctly determined by this method. The API Staph-Ident system was more accurate in correctly identifying Staphylococcus aureus (93.9%) than in correctly identifying non-S. aureus species (41.8%). False identifications of Staphylococcus epidermidis and Staphylococcus hominis were the main reasons for the incorrect identifications of the non-S. aureus species.

Multiple antibiotic resistance in fecal, cecal and colonic coliforms from pigs fed therapeutic and subtherapeutic concentrations of chlortetracycline

Multiple resistance and antibiotic resistance were examined in coliforms isolated at slaughter from the large intestines of two groups of growing pigs that had received antibiotic-free diets since weaning. One group of pigs was from an antibiotic-fed (AB) herd that routinely received chlortetracycline (CTC) in feed, while the other group was from a nonantibiotic-fed (NAB) herd that had not received antibiotics for 8 yr. After a 20-d adjustment period in a common production facility, the mean number of antibiotics in the resistant pattern of isolates from NAB pigs was found to be lower (P less than .05) than that in isolates from AB pigs. The proportions of isolates resistant to ampicillin, kanamycin, neomycin, penicillin, streptomycin and sulfamethizole were lower (P less than .05) in NAB pigs than in AB pigs. Similar herd differences were not observed after pigs from both herds were maintained in a common production facility on an antibiotic-free diet for 105 d. Oral administration of CTC at therapeutic (220 micrograms/g of diet) and at subtherapeutic (27.5 micrograms/g) levels for 14 d increased (P less than .05) the mean numbers of antibiotics in the resistance patterns of isolates from NAB pigs, but did not alter multiple resistance in isolates from AB pigs. After 14 d, subtherapeutic CTC increased the percentage of isolates resistant to ampicillin, kanamycin, penicillin, streptomycin, tetracycline and sulfamethizole, while therapeutic CTC only increased the percentage of isolates from NAB pigs resistant to penicillin, tetracycline and sulfamethizole. Similar antibiotic effects were not seen in isolates from AB pigs after 14 d and were not seen in isolates from either group of pigs at the end of an 85-d feeding trial.

Effect of feeding chlortetracycline or virginiamycin on shedding of salmonellae from experimentally-infected swine

Swine from a herd routinely fed subtherapeutic levels of chlortetracycline (CTC) were fed a diet containing 55 mg of CTC/kg, a diet containing 55 mg of virginiamycin/kg, or a control diet. All animals were inoculated with Salmonella typhimurium that was susceptible to tetracycline. The quantity, duration and prevalence of shedding of S. typhimurium were determined. The infecting organism was first recovered from the animals fed CTC or the control diet on d 2, from animals fed virginiamycin on d 7 and from animals in a second control group on d 10. The infecting organism was recovered in fewer samples obtained during the initial 7 d postinfection than in those obtained during the last 24 d of the study. Little transfer of resistance to the infecting organism seemed to have occurred from the resident microflora because only two isolates (1%) had resistant patterns that differed from that of the infecting organism. Feeding CTC or virginiamycin to swine did not significantly increase or prolong shedding of an experimentally infected tetracycline-susceptible strain of S. typhimurium. Neither antibiotic affected the drug resistance of the infecting organism.

Milking frequency as related to udder health and milk production

Twenty-four cows each in two trials were paired on age, stage of lactation, and projected milk production to compare the incidence of mastitis with milking two or three times per day over 12 wk. Teats of all cows were dipped daily in a suspension of Streptococcus agalactiae (one of two strains). Average daily milk production for the combined trials was 24.0 kg for the thrice daily group and 22.0 kg for the twice daily group. Quarter foremilk samples were taken every 2 wk for somatic cell counts and bacterial isolations. Somatic cell counts of cows milked twice per day averaged 31 x 10(3) per milliliter compared to 26 x 10(3) for three milkings. There were no significant differences in numbers of new bacterial infections between groups. Cows milked twice per day had 41 new infections and 7 clinical infections and the three-time group had 39 new infections and 2 clinical infections. The many corynebacteria infections were related to the experimental design in which no disinfectant teat dip was used. Three milkings per day did not affect udder health.

Influence of bilateral nephrectomy on selected gastrointestinal bacteria in the rat

A series of experiments was designed to investigate the influence of acute renal failure on selected gastrointestinal bacteria. Sprague-Dawley male rats were bilaterally nephrectomized to induce acute renal failure, with sham-operated animals serving as controls. After 48 h animals were sacrificed and the stomachs, upper and lower small intestines, ceca, and colons were excised and subjected to microbial analyses. Lactobacilli and streptococci including enterococci, were 1-2 log counts higher in the stomachs of anephric rats than those of sham-operated controls; lactobacilli were increased similarly in the upper small intestines of these animals. Coliforms including Escherichia coli, and Proteus were 1-2 log counts higher in the lower small intestine of anephric rats than those of sham-operated rats. The decreased gastric pH, increased cecal pH, hypothermia, and delayed gastric emptying observed in nephrectomized rats could partly explain the different microfloras in these animals.

Psychrotrophic Bacteria Reduce Cheese Yield 1

Psychrotrophic strains of Bacillus and Pseudomonas that demonstrated both proteolytic and lipolytic activity were incubated with Grade A milk. The yield of direct-acid cheese manufactured from inoculated milk decreased as psychrotrophic inoculation level increased. Yield reduction resulted from both lipid and protein degradation, and accounted for approximately 45 and 55% of the dry matter loss, respectively. Fat losses were observed from decreased milkfat tests and increased acid degree values. Protein losses were observed from increased non-protein nitrogen and whey nitrogen values. Therefore, cheese yield studies must involve assays of both protein and lipid on a dry matter basis. Acid degree values and fat disappearance in stored milk and total nitrogen in whey were the best indicators of reduction in yields. Although bacterial enumeration, titratable acidity and pH were not good indicators of yield, they may be important in determining when yield loss starts.

Effect of Pre-Cure Freezing and Thawing on the Microflora, Fat Characteristics and Palatability of Dry-Cured Ham 1

Hams were placed in cure after thawing by 3 methods: at 2C, at 16C, and in water at 37C. A fourth group was placed in cure while still frozen. Microbiological populations and fat rancidity tests were determined at various intervals during processing. Sensory scores and tenderness values were determined after 3 months of aging. Clostridium perfringens , Bacillus cereus , Escherichia coli , coliforms and enterococci were not detected after salt equalization. Hams cured without thawing had lower initial bacterial, yeast and mold counts but no differences among thaw groups were observed in counts during aging. Hams thawed in water had lower flavor and overall satisfaction scores than the other groups. Fat breakdown as noted by FFA, TBA and peroxide values increased with aging but were erratic although ham cured without thawing had lower peroxide values. Satisfactory dry-cured aged hams were produced regardless of method of thawing. However, since hams cured without thawing had less weight loss, lower peroxide numbers, lower initial microbial counts and similar final microbial counts and sensory scores, it appears that hams do not need to be thawed to produce dry-cured aged hams.

Effects of Pre-cure Storage and Smoking on the Microflora and Palatability of Aged Dry-cured Hams 1

Forty-two similar weight hams were held 3 and 6 days postmortem at 2 C, dry-cured, and held at 13 C for 4 weeks for salt equalization. Half were then smoked and half were not smoked. All hams were aged for 3 months at 24 C. Six hams from each group were selected for external microbial evaluation and three were selected for internal microbial evaluation before curing, after curing, after salt equalization or smoking, and after aging 1, 2 and 3 months. Aerobic counts were higher in hams held 6 days than in those held 3 days but the difference decreased as processing time progressed. Smoking decreased surface counts. Enterococci, Bacillus cereus and fluorescent pseudomonads counts were initially low and these organisms were virtually absent after aging 1 month. Coliforms were initially low and were not detected after salt equalization. Staphylococci increased through 3 months of aging with very few isolates being coagulase-positive. Yeasts and molds increased gradually through processing and aging. No Clostridium perfringens or Salmonella were detected in uncured hams. There were no significant differences in palatability traits due either to pre-cure holding time or smoking.

Milking vacuum influencing indicators of udder health

In 23 first-calf heifers we ascertained effects that milking vacuums of 34, 42.5, and 51 kPa had on udder health by percentage of quarter-samples positive for Staphylococcus aureus, total counts of bacteria on blood agar, teat end scores, California Mastitis Test, and direct microscopic somatic cell count. Measures were higher for 51 kPa than for 34 or 42.5 kPA. Samples from cows milked at 42.5 kPa had a lower percentage of samples positive for Staphylococcus aureus, lower total count, lower California Mastitis Test, and lower direct microscopic somatic cell count than samples from cows milked with 34 kPa. Test end scores increased with increase in milking vacuum. Approximately 49% of the samples from cows milked with 51 KPa were positive for Staphylococcus aureus compared with 19.9 and 16.7% for 34 and 42.5 kPa. Milking vacuum of 51 kPa resulted in the highest percentage of cows with three (44.5%) and four (24.1%) quarters positive for Staphylococcus aureus. A milking vacuum of 51 kPa could increase mastitis in a herd having Staphylococcus aureus as the prevalent microorganism causing mastitis.

Variation of Laboratory Cheesemaking Procedures

The variability of yield for Cheddar cheese manufactured in 7- and 390-kg-size vats and cottage and direct-acid-set cheese manufactured in 7-kg-size vats was compared. The direct-acid-set cheese had less variability than Cheddar or cottage cheese (% coefficient of variation was .91, 1.79 and 4.69, respectively) manufactured in 7-kg-size vats and Cheddar cheese manufactured in 360-kg-size vats (% coefficient of variation was .91 and 1.98, respectively). Cottage cheese had the largest variability between replications using small vats because of additional stirring and curd manipulation required in its manufacture. Therefore, when cheese variety is of no concern, the direct-acid-set cheese procedure would reduce variation between replication more than traditional Cheddar procedures when used in an experimental design. In addition, a greater number of vats of cheese could be manufactured per day using the small vat system.

Effect of Five Surface Area-to-Sample Volume Ratios During Preliminary Incubation on The Bacterial Count of Raw Milk 1, 2

Effects of ratio of surface area to volume of sample during preliminary incubation (PI) and of different plate incubation temperatures on bacterial counts of raw milk samples were studied. One hundred and twenty Grade A raw milk samples collected during a 10-month period were divided into five 100-ml aliquots and allotted to one of five surface area to volume ratios. The ratios during PI ranged from 0 cm²/100 ml to 149.74 cm²/100 ml. Following PI, pour plates of each treatment were incubated at 26, 30 and 32 C for 72 h. The Standard Plate Counts (SPC) ranged from 89 × 10¹ to 20 × 10⁸/ml, with the SPC of 73.6% of the samples being less than 1 × 10⁵/ml. Counts after PI tended to be higher as the plate incubation temperature decreased from 32 to 26 C and as the ratio of surface area to volume of sample increased. None of the differences between the counts for the 15 treatment-incubation temperatures was significant. Counts of 61 samples increased less than one log count during PI, while counts of 33 and 16 samples increased one to two log counts and over two log counts, respectively. The greater the SPC, the smaller the increase in count during PI. Of the 81 samples with SPC less than 1 × 10⁵/ml, 29 had counts after PI that exceeded 2 × 10⁵/ml.

Fate of Selected Pathogens Inoculated into Foods Prepared in Slow Cookers 1, 2

Staphylococcus aureus , Clostridium perfringens . Salmonella choleraesuis , and Salmonella typhimurium were inoculated (10⁸ cells or spores) into two slow cookers containing green bean casserole, baked navy beans, chicken cacciatore, barbecued ribs or pork pot roast, and their fate determined after cooking. Heating patterns also were determined at three positions inside the two cookers. None of the foods cooked in either of the slow cookers contained detectable levels of S. aureus or salmonellae. The similarity between C. perfringens vegetative and spore counts indicate that only spores were present in the cooked foods. Except for the green bean casserole cooked using a low temperature setting, cooking resulted in a 0.44-1.67 and 0.36-1.54 log count reduction, respectively, of vegetative cells and spores of C. perfringens . Counts of vegetative cells and spores after cooking the green bean casserole were approximately .18 and .30 log counts higher than the uncooked counts. The mean times for the coldest areas in Cooker A to reach 50 C were 2.57 and 0.97 h, respectively, for the low (80 watts) and high (160 watts) temperature settings. The mean times for the coldest areas in Cooker B (removable liner) to reach 50 C were 2.35 and 0.52 h for the low (130 watts) and high (260 watts) temperature settings, respectively. Results suggest that when the recommended quantities of ingredients are used and the proper cooking procedure followed, foods prepared in the slow cookers studied do not present a health hazard.

Comparing Three Methods for Counting Raw-Milk Pour Plates 1

Six hundred Grade A raw milk samples were plated on Standard Methods agar and the pour-plate counts compared using three methods. Counts of each sample were determined after incubation at 26, 30 and 32 C for 72 h with an automatic colony counter (ACC). Quebec colony counter and hand tally (HT) and Quebec colony counter and electronic probe with digital register (DT). No significant differences were found between the counts obtained with the HT and the DT for any of the three incubation temperatures. The ACC counts were lower (P < .01) than the manual counts at each temperature. The ACC counts were .39, .35 and .31 log counts lower than the manual count at 26, 30 and 32 C, respectively. The ACC counts at 72 h were higher (P < .01) than the ACC counts at 48 h for all temperatures. The difference was about .41 log counts. The two manual counts were totally correlated, while the correlation between the ACC counts and the manual count; was 97. Regression equations were formulated for predicting the 72 h HT count from either the 48 or 72 h ACC count.

Effect of Heat Treatments on Survival and Growth of a Psychrotroph and on Nitrogen Fractions in Milk 1

Grade A raw milk which had initial psychrotrophic counts of less than 10³/ml was inoculated with an antibiotic-resistant Pseudomonas sp. to a final cell concentration of 10², 10⁴, or 10⁶/ml. The inoculated milk was held at 4 C for 14 h and then exposed to the following time-temperature treatments: 72 C for 15 sec, 79 C for 15 sec, 88 C for 10 sec, and 95 C for < 5 sec. An uninoculated raw milk control was handled and analyzed along with inoculated samples. Aliquots of milk were analyzed for marked Pseudomonas sp., total psychrotrophic counts, numbers of Pseudomonas , and for distribution of nitrogen before and after each heat treatment and after storage of non-heat-treated raw milk and heat-treated samples for 7 and 14 days at 7 C. Psychrotrophic counts were significantly affected by heat treatment, initial cell inoculum, days stored, and plating media. Non-casein N, non-casein protein, total albumin, β-lactoglobulin, proteose-peptone, and globulin N were significantly decreased by heat treatment. Non-casein N, non-casein protein, β-lactoglobulin, and proteose-peptone were significantly increased by days of storage.

Microflora of Retail Fluid Milk Products 1

Numbers and types of microorganisms in retail pasteurized fluid milk products were determined as well as the effect that type of product, brand, and season of the year had on counts of 13 different microbial types. Clostridium perfringens was the only pathogen detected and it averaged less than one organism per milliliter. Chocolate milk samples generally had the highest mean counts, followed by skim milk, low-fat (2%), and whole milk (3.25%). Most brands had means for the various microbial counts which were not significantly different from each other. Only three brands had counts which differed significantly from other brands. Psychrotrophic, coliform, staphylococcal, yeast and mold, and Standard Plate Counts were highest between May and October, while counts for spores, streptococci, and thermophiles were highest between December and March. No seasonal trends were detected for counts of anaerobes, C. perfringens , enterococci, or lactobacilli.

Effect of Various Combinations of Medium, Diluent and Incubation Conditions on Recovery of Bacteria from Manufacturing Grade and Grade A Raw Milk 1

Recovery of microorganisms from manufacturing grade and Grade A raw milk was determined using 18 plating combinations which consisted of three media, three diluents, and two incubation temperatures. Plating conditions specified in Standard Methods for doing the Standard Plate Count was one of the 18 combinations used. Combinations studied consisted of Standard Methods Agar, Schaedler Agar, and Eugonagar as plating medium; phosphate buffered distilled water, 0.1% peptone water, and Ringer solution as diluent; and 28 C for 72 h and 32 C for 48 h as incubation temperature. Forty manufacturing grade and 40 Grade A raw milk samples were plated using each of the 18 combinations. Highest mean counts were obtained for both grades of milk with the combination of Standard Methods Agar, phosphate buffered distilled water, and 28 C for 72 h. Samples, diluents, media, and samples × diluents interaction had a highly significant (P<.01) effect on counts of manufacturing grade milk samples; while samples, media, and temperatures had a highly significant (P<.01) effect on counts of the Grade A raw milk samples. Nonsignificant differences were obtained in counts of the manufacturing gradesamples with eight of 17 plating combinations when compared with counts obtained with Standard Methods. Counts for Grade A samples obtained with six of 17 combinations were similar to counts obtained with Standard Methods.