Molecular analysis of ampicillin-resistant sporadic Salmonella typhi and Salmonella paratyphi B clinical isolates

Objective: To study the mechanisms of antibiotic resistance in Salmonella typhi and Salmonella paratyphi B clinical isolates, and the clonality of resistant strains.
Method: Antibiotic susceptibility was tested by disk-agar diffusion. Conjugation experiments and plasmid analysis by agarose gel electrophoresis after Eco RI digestion were followed by hybridization to a digoxigenin-labeled TEM-type β-lactamase probe. DNA fingerprints were obtained by pulsed-field gel electrophoresis of Xba I-digested chromosomal DNA.
Results: Three S. typhi isolates (7% of the isolates studied), of which one was ampicillin resistant and the other two multiresistant (ampicillin, chloramphenicol, tetracycline, sulfamethoxazole/trimethoprim and streptomycin), and two ampicillin-resistant S. paratyphi B isolates (25% of the isolates studied) were further evaluated. A 34-MDa conjugative plasmid, previously isolated from Salmonella enteritidis , conferred ampicillin resistance. A 100-MDa conjugative plasmid encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim, as well as ampicillin. Chromosomal fingerprinting revealed two distinct resistant strains for each serovar which were different from a matched set of sensitive S. typhi strains.
Conclusions: Two conjugative, TEM-type β-lactamase-encoding plasmids conferred ampicillin resistance to S. typhi and S. paratyphi B. The 34-MDa plasmid was identical to that previously characterized from S. enteritidis , while the 100-MDa plasmid also encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim. Resistant isolates did not belong to a single clone but rather represented distinct strains.     

Characterization of endemic Shigella flexneri strains in Somalia: antimicrobial resistance, plasmid profiles, and serotype correlation.

One hundred twelve Shigella flexneri strain isolated from children with diarrheal disease in Somalia in 1983, 1984, 1988, and 1989 were analyzed for serotype, plasmid profile, and genetic location of antimicrobial resistance determinants. The prevalent serotypes were 4 (46% of the isolates), 1b (16%), 2a (16%), 3a (12%), and 6 (8%). Each serotype was associated with a characteristic predominant plasmid profile, whereas no specific correlation between antimicrobial resistance patterns and single serotypes was found. All but three of the strains were resistant at least to ampicillin, chloramphenicol, spectinomycin, and tetracycline. Of these resistant strains, 41 were resistant to sulfonamide and streptomycin and 14 were resistant to trimethoprim or trimethoprim and kanamycin. The genes for resistance to ampicillin, chloramphenicol, spectinomycin, and tetracycline formed a linkage group located on the chromosome of the strains of all serotypes. The genes for resistance to sulfonamide and streptomycin were located on a 6.3-kb plasmid in strains of serotypes 1b, 2a, and 4. Conjugative trimethoprim or trimethoprim and kanamycin resistance plasmids with lengths of 80 to 110 kb were present in strains of serotypes 1b, 2a, 3a, and 4. The systematic presence of a chromosomal component in this uncommon genetic plasmid-chromosome configuration may play a role in the emergence of increased genetic stability of resistance patterns in S. flexneri.     

Spontaneous deletions of drug-resistance determinants from Salmonella typhimurium in Escherichia coli

Plasmids isolated from two different clinical isolates of Salmonella typhimurium, both resistant to the antibiotics ampicillin, tetracycline, streptomycin and chloramphenicol, were used to transform Escherichia coli. Segregation of antibiotic-resistance determinants occurred in both cases. Analysis of plasmids from one set of segregants by DNA-DNA hybridisation indicated that the segregation was due to precise deletions in the transforming plasmid.     

Bacteremia caused by a lactose-fermenting, multiply resistant Salmonella typhi strain in a patient recovering from typhoid fever.

A female patient suffered a typical attack of typhoid fever due to a lactose-negative, fully susceptible Salmonella typhi strain. During convalescence she became febrile, and a lactose-fermenting S. typhi strain resistant to ampicillin, chloramphenicol, tetracycline, and trimethoprim was isolated from blood culture. This isolated was shown to harbor a plasmid which cotransferred lactose fermentation and antibiotic resistance.     

Antimicrobial resistance of Salmonella isolates from swine

We examined the antimicrobial resistance of 1,257 isolates of 30 serovars of Salmonella enterica subsp. enterica isolated from swine. Serovars Typhimurium and Typhimurium var. Copenhagen were widespread and were frequently multidrug resistant, with distinct resistance to ampicillin, kanamycin, streptomycin, sulfamethoxazole, and tetracycline and to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline, respectively.     

Characterisation of methicillin-resistant Staphylococcus aureus from Kuwait hospitals with high-level fusidic acid resistance

Forty-seven fusidic acid- and methicillin-resistant Staphylococcus aureus isolates from clinical samples in four hospitals in Kuwait were studied for their relatedness by biotyping and pulsed-field gel electrophoresis (PFGE) and for the genetic location of their resistance determinants. Forty-four isolates were resistant to gentamicin, kanamycin and neomycin. Forty-one isolates were resistant to erythromycin and trimethoprim, 10 were resistant to chloramphenicol and four were resistant to ciprofloxacin. They contained two or three plasmids of c. 28, 2.8 and 1.8 kb. Genetic studies demonstrated that resistance to cadmium, propamidine isethionate and ethidium bromide were linked and were carried on the c. 28-kb plasmid. Chloramphenicol resistance was encoded by the 2.8-kb plasmid in resistant isolates. No resistance was associated with the 1.8-kb plasmid and this was considered to be a cryptic plasmid. Resistance to fusidic acid, methicillin, benzylpenicillin, gentamicin, kanamycin, neomycin, tetracycline, trimethoprim, erythromycin and ciprofloxacin were located on the chromosome. All the isolates produced urease, but varied in the production of haemolysins, pigments, lipase and lecithinase and were classified into nine biotypes. In contrast, PFGE divided the isolates into two major patterns with one PFGE type constituting the majority of isolates in all four hospitals. The presence of the dominant PFGE pattern in all four hospitals suggests that it is an epidemic MRSA clone with the capacity to spread. Infection control measures should be directed towards restricting the further spread of this clone.     

Transfer of resistance determinants from a multi-resistant Staphylococcus aureus isolate

The clinical isolate Staphylococcus aureus WBG1024 was resistant to cadmium, benzyl penicillin, kanamycin, neomycin, streptomycin, tetracycline and trimethoprim and harboured a conjugative plasmid pWBG637 (34.5 kb) and non-conjugative plasmids of 23.8, 4.4, 2.8 and 1.9 kb. Transduction and mixed-culture transfer experiments demonstrated that the 4.4-kb plasmid (pWBG632) encoded resistance to tetracycline and the 23.8-kb plasmid (pWBG628) encoded resistance to cadmium, benzyl penicillin, kanamycin, neomycin and streptomycin. The conjugative plasmid pWBG637 was able to mobilise a further 4.4-kb plasmid (pWBG633) encoding streptomycin resistance and recombined with the multiresistance plasmid pWBG628 to produce transconjugantes of various resistance phenotypes.     

Detection of multidrug-resistant Salmonella enterica serovar typhimurium phage types DT102, DT104, and U302 by multiplex PCR

Salmonella enterica serovar Typhimurium is a common cause of nontyphoidal salmonellosis in humans and animals. Multidrug-resistant serovar Typhimurium phage type DT104, which emerged in the 1990s, has become widely distributed in many countries. A total of 104 clinical isolates of Salmonella serogroup B were collected from three major hospitals in Taiwan during 1997 to 2003 and were examined by a multiplex PCR targeting the resistance genes and the spv gene of the virulence plasmid. A total of 51 isolates (49%) were resistant to all drugs (ACSSuT [resistance to ampicillin, chloramphenicol, streptomycin, sulfonamide, and tetracycline]), and all contained a 1.25-kb PCR fragment of integron that is part of the 43-kb Salmonella genomic island 1 (SGI1). The second group was resistant to SSu (28%), and the third was susceptible to all five drugs (13%). Fifty-nine isolates were serotyped to be serovar Typhimurium by the tube agglutination method using H antisera. The virulence plasmid was found in 54 (91.5%) of the 59 serovar Typhimurium isolates. A majority (94.1%) of the Salmonella serogroup B isolates with the ACSSuT resistance pattern harbored a virulence plasmid. Phage typing identified three major phage types: DT104, DT120, and U302. Analysis of the isolates by pulsed-field gel electrophoresis showed six genotypes. We found two genotypes in DT104 strains, two in DT120, and the other two in U302. The presence of a monophasic serovar (4,5,12:i:-) has added difficulty in the determination of the serovars of multidrug-resistant Salmonella serogroup B isolates. Nevertheless, the multiplex PCR devised in the present study appears to be efficient and useful in the rapid identification of ACSSuT-type serovar Typhimurium with SGI1, irrespective of their phage types.     

Characterization of Salmonella enterica serotype Typhimurium isolates from human, food, and animal sources in the Republic of Ireland

A potential epidemic clone of Salmonella enterica serotype Typhimurium DT104, and the possible emergence of S. enterica serotype Typhimurium DT104b, has been identified from the characterization of 67 S. enterica serotype Typhimurium strains from three sources, human gastroenteritis isolates, isolates from food samples, and veterinary isolates, by antimicrobial resistance profiling, phage typing, and pulsed-field gel electrophoresis. Resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline was found in 77.6% of these strains.     

Molecular characterization of multidrug-resistant Salmonella enterica subsp. enterica serovar Typhimurium isolates from swine

As part of a longitudinal study of antimicrobial resistance among salmonellae isolated from swine, we studied 484 Salmonella enterica subsp. enterica serovar Typhimurium (including serovar Typhimurium var. Copenhagen) isolates. We found two common pentaresistant phenotypes. The first was resistance to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (the AmCmStSuTe phenotype; 36.2% of all isolates), mainly of the definitive type 104 (DT104) phage type (180 of 187 isolates). The second was resistance to ampicillin, kanamycin, streptomycin, sulfamethoxazole, and tetracycline (the AmKmStSuTe phenotype; 44.6% of all isolates), most commonly of the DT193 phage type (77 of 165 isolates), which represents an unusual resistance pattern for DT193 isolates. We analyzed 64 representative isolates by amplified fragment length polymorphism (AFLP) analysis, which revealed DNA fingerprint similarities that correlated with both resistance patterns and phage types. To investigate the genetic basis for resistance among DT193 isolates, we characterized three AmKmStSuTe pentaresistant strains and one hexaresistant strain, which also expressed resistance to gentamicin (Gm phenotype), all of which had similar DNA fingerprints and all of which were collected during the same sampling. We found that the genes encoding the pentaresistance pattern were different from those from isolates of the DT104 phage type. We also found that all strains encoded all of their resistance genes on plasmids, unlike the chromosomally encoded genes of DT104 isolates, which could be transferred to Escherichia coli via conjugation, but that the plasmid compositions varied among the isolates. Two strains (strains UT08 and UT12) had a single, identical plasmid carrying bla(TEM) (which encodes ampicillin resistance), aphA1-Iab (which encodes kanamycin resistance), strA and strB (which encode streptomycin resistance), class B tetA (which encodes tetracycline resistance), and an unidentified sulfamethoxazole resistance allele. The third pentaresistant strain (strain UT20) was capable of transferring by conjugation two distinct resistance patterns, AmKmStSuTe and KmStSuTe, but the genes were carried on plasmids with slightly different restriction patterns (differing by a single band of 15 kb). The hexaresistant strain (strain UT30) had the same plasmid as strains UT08 and UT12, but it also carried a second plasmid that conferred the AmKmStSuGm phenotype. The second plasmid harbored the gentamicin resistance methylase (grm), which has not previously been reported in food-borne pathogenic bacteria. It also carried the sul1 gene for sulfamethoxazole resistance and a 1-kb class I integron bearing aadA for streptomycin resistance. We also characterized isolates of the DT104 phage type. We found a number of isolates that expressed resistance only to streptomycin and sulfamethoxazole (the StSu phenotype; 8.3% of serovar Typhimurium var. Copenhagen strains) but that had AFLP DNA fingerprints similar or identical to those of strains with genes encoding the typical AmCmStSuTe pentaresistance phenotype of DT104. These atypical StSu DT104 isolates were predominantly cultured from environmental samples and were found to carry only one class I integron of 1.0 kb, in contrast to the typical two integrons (InC and InD) of 1.0 and 1.2 kb, respectively, of the pentaresistant DT104 isolates. Our findings show the widespread existence of multidrug-resistant Salmonella strains and the diversity of multidrug resistance among epidemiologically related strains. The presence of resistance genes on conjugative plasmids and duplicate genes on multiple plasmids could have implications for the spread of resistance factors and for the stability of multidrug resistance among Salmonella serovar Typhimurium isolates.     

Evaluation of minimum inhibitory concentration of quinolones and third generation cephalosporins to Salmonella typhi isolates

A total of 323 Salmonella typhi isolates (261 isolates obtained during 1995-99 from Ludhiana and 62 randomly selected isolates obtained between 1980-99 from Chandigarh) were analyzed for drug resistant pattern. S. typhi isolates prior to 1986 were sensitive to all the antimicrobials tested by disc diffusion method. Most common multidrug resistance pattern noticed was ACCo T i.e. resistance to ampicillin, Chloramphenicol, cotrimoxazole and tetracycline. This study has revealed that withdrawal of chloramphenicol due to high level of resistance during 1990-94, has led to re-emergence of 43-93 percent chloramphenicol sensitive mutants during 1995-99. Two S. typhi isolates in 1995 and one in 1999 from Ludhiana depicted resistance to ciprofloxacin. Susceptibility of S. typhi isolates to third generation cephalosporins ranged between 87 to 100 per cent. There was a gradual increase with time period in mean minimum inhibitory concentration (MIC) of ciprofloxacin as it increased from 0.066 ug/ml for 1980-83 S. typhi isolates to 0.13 ug/ml for the 1996-99 isolates. Similarly, cefotaxime mean MIC for 1980-83 isolate was 0.172 ug/ml which further increased up to 0.32 ug/ml for S. typhi isolates encountered between 1996-99. In contrast, mean MIC value of 0.62 ug/ml of Ceftriaxone remained unchanged irrespective of the year of isolation of S. typhi isolates.     

Analysis of antimicrobial resistance genes detected in multidrug-resistant Salmonella enterica serovar Typhimurium isolated from food animals

Multidrug-resistant (MDR) Salmonella enterica serovar Typhimurium is the most prevalent penta-resistant serovar isolated from animals by the U.S. National Antimicrobial Resistance Monitoring System. Penta-resistant isolates are often resistant to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline. To investigate MDR in Salmonella Typhimurium (including variant 5-), one isolate each from cattle, poultry, and swine with at least the ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline phenotype were selected for each year from 1997 to 2007 (n = 33) for microarray analysis of antimicrobial resistance, incompatibility IncA/C, and HI1 plasmid genes. Cluster analysis based on these data separated 31 of the isolates into two groups A and B (15 and 16 isolates, respectively). Isolates in group A were phage type DT104 or U302 and were mostly swine isolates (7/15). Genes detected included intI1, bla(PSE-1), floR, aadA, sulI, tet(G), and tetR, which are often found in Salmonella Genomic Island I. Isolates in group B had numerous IncA/C plasmid genes detected and were mostly cattle isolates (9/16). Genes detected included bla(CMY-2), floR, aac(3), aadA, aphA1, strA, strB, sulI, sulII, dfrA, dhf, tet(A)(B)(C)(D), and tetR, which are often found on MDR-AmpC IncA/C plasmids. The IncA/C replicon was also detected in all group B isolates. The two remaining isolates did not cluster with any others and both had many HI1 plasmid genes detected. Linkage disequilibrium analysis detected significant associations between plasmid replicon type, phage type, and animal source. These data suggest that MDR in Salmonella Typhimurium is associated with DT104/Salmonella Genomic Island I or IncA/C MDR-AmpC encoding plasmids and these genetic elements have persisted throughout the study period.     

Characterization of Salmonella enterica serovar typhimurium DT104 isolated from Denmark and comparison with isolates from Europe and the United States

A total of 136 isolates of Salmonella enterica serovar Typhimurium DT104 from Denmark (n = 93), Germany (n = 10), Italy (n = 4), Spain (n = 5), and the United Kingdom (n = 9) were characterized by antimicrobial resistance analysis, plasmid profiling, pulsed-field gel electrophoresis (PFGE) with the restriction enzymes XbaI and BlnI, and analysis for the presence of integrons and antibiotic resistance genes. The isolates from Denmark were from nine pig herds, while the isolates from other countries were both of animal and of human origin. All but 10 isolates were resistant to ampicillin, chloramphenicol, spectinomycin, streptomycin, sulfonamides, and tetracycline. Five isolates from the United Kingdom and Spain were sensitive to all antibiotics examined, whereas four isolates from the United Kingdom and the United States were also resistant to one or more of the antibiotics, namely, gentamicin, neomycin, and trimethoprim. All but two strains had the same PFGE profiles when the XbaI restriction enzyme was used, while seven different profiles were observed when the BlnI restriction enzyme was used. Different dominating BlnI types were observed among European isolates compared with the types observed among those from the United States. All the isolates harbored common 95-kb plasmids either alone or in combination with smaller plasmids, and a total of 11 different plasmid profiles were observed. Furthermore, all but one of the multidrug-resistant isolates contained two integrons, ant (3")-Ia and pse-1. Sensitive isolates contained no integrons, and isolates that were resistant to spectinomycin, streptomycin, and sulfonamides had only one integron containing ant (3")-Ia. When restriction enzyme BlnI was used, the 14 isolates from one of the nine herds in Denmark showed unique profiles, whereas isolates from the remaining herds were homogeneous. Among isolates from seven of nine herds, the same plasmid profile (95 kb) was observed, but isolates from two herds had different profiles. Thus, either PFGE (with BlnI) or plasmid profiling could distinguish isolates from three of nine pig herds in Denmark. The epidemiological markers (antimicrobial susceptibility testing, plasmid profiling, and PFGE) applied demonstrated high in vivo stability in the Danish herds. This may indicate that some different strains of multidrug-resistant S. enterica serovar Typhimurium DT104 have been introduced into Danish food animal herds. The presence of isolates from six different countries with similar profiles by PFGE with XbaI and highly homogeneous profiles by PFGE with BlnI indicate that multidrug-resistant S. enterica serovar Typhimurium DT104 has probably been spread clonally in these countries. However, some minor variation could be observed by using plasmid profiling and profiling by PFGE with BlnI. Thus, a more sensitive technique for subtyping of strains of DT104 and a broader investigation may help in elucidating the epidemiological spread of DT104 in different parts of the world.     

Detection of <Emphasis Type="Italic">Salmonella enterica</Emphasis> serovar Typhimurium with pUO-StVR2-like virulence-resistance hybrid plasmids in the United Kingdom

The aim of this study was to investigate the presence in the United Kingdom (UK) of Salmonella enterica serovar Typhimurium isolates carrying pUO-StVR2-like virulence-resistance hybrid plasmids that originated from pSLT. One hundred and fifty ampicillin-resistant isolates of S. Typhimurium, collected in different regions of the UK during 2006, were screened for the presence of bla _OXA-1 carried by an InH-like integron (2000 bp/bla _OXA-1-aadA1) characteristic of pUO-StVR2. Positive isolates were tested for the presence of a large plasmid that hybridised with probes specific for the bla _OXA-1 and spvC genes, used as resistance and virulence markers of the hybrid plasmid, respectively. Eleven out of the 150 isolates fulfilled both criteria and were assigned to the S. Typhimurium pUO-StVR2 group. Nine were resistant to ampicillin, chloramphenicol, streptomycin/spectinomycin, sulfonamides and tetracycline, encoded by bla _OXA-1, catA1, aadA1-like, sul1 and tet(B), respectively, and carried a pUO-StVR2-like plasmid of ca. 130 kb. Two contained hybrid plasmids of smaller size and lacked resistance(s) to chloramphenicol or chloramphenicol and tetracycline. The eleven isolates, which showed five and six closely related XbaI and BlnI profiles, respectively, were resistant to nitrofurantoin. In conclusion, multidrug-resistant S. Typhimurium isolates of the pUO-StVR2 group, which are endemic in Spain, were also detected in the UK, albeit with a low frequency (7.3%).     

Antimicrobial resistance of Salmonella serotypes isolated from slaughter-age pigs and environmental samples

The aim of this study was to determine the antimicrobial resistance patterns of Salmonella strains isolated from slaughter-age pigs and environmental samples collected at modern swine raising facilities in Brazil. Seventeen isolates of six serotypes of Salmonella enterica subsp. enterica were isolated out of 1,026 collected samples: Salmonella Typhimurium (1), Salmonella Agona (5), Salmonella Sandiego (5), Salmonella Rissen (1), Salmonella Senftenberg (4), and Salmonella Javiana (1). Resistance patterns were determined to extended-spectrum penicillin (ampicillin), broad-spectrum cephalosporins (cefotaxime and ceftriaxone), aminoglycosides (streptomycin, neomycin, gentamicin, amikacin, and tobramycin), narrow-spectrum quinolone (nalidixic acid), broad-spectrum quinolone (ciprofloxacin and norfloxacin), tetracycline, trimethoprim, and chloramphenicol. Antimicrobial resistance patterns varied among serotypes, but isolates from a single serotype consistently showed the same resistance profile. All isolates were resistant to tetracycline, streptomycin, and nalidixic acid. One isolate, Salmonella Rissen, was also resistant to cefotaxime and tobramycin. All serotypes were susceptible to ceftriaxone, norfloxacin, ciprofloxacin, ampicillin, gentamicin, and chloramphenicol. The high resistance to tetracycline and streptomycin may be linked to their common use as therapeutic drugs on the tested farms. No relation was seen between nalidixic acid and fluoroquinolone resistance.     

Antibiotic resistance of strains of Vibrio cholerae eltor isolated in Douala (Cameroon)

Endemic cholera has been prevalent in Douala since 1972, with sudden epidemic outbreaks occurring every two years during the dry season. The massive and systematic use of chemoprophylaxis since April, 1983 has led to the selection of strains of Vibrio cholerae eltor that are resistant to sulphamide and tetracycline. During the 1984-1985 epidemic, 89.3% of the isolated strains were resistant to sulphamides, 87.5% to a sulfamethoxazole-trimethoprim combination and to the 0/129 disk, 55.3% to tetracycline, 91.1% to chloramphenicol, 73.2% to streptomycin and 94.6% to ampicillin. The epidemic aspect of this multiple resistance to antibiotics raises the issue of the role of a group C incompatibility resistance plasmid. As regards prophylaxis, until hygiene conditions can be improved, which is the only way cholera can be eradicated from our region, vaccination with oral vaccines such as that of the Institut Pasteur seems to be the best way of preventing further epidemics.     

Endemic, epidemic clone of Salmonella enterica serovar typhi harboring a single multidrug-resistant plasmid in Vietnam between 1995 and 2002

Salmonella enterica serovar Typhi strains resistant to ampicillin, chloramphenicol, tetracyclines, streptomycin, and cotrimoxazole, isolated from sporadic cases and minor outbreaks in Vietnam between 1995 and 2002, were typed and compared. Plasmid fingerprinting, Vi bacteriophage typing, XbaI pulsed-field gel electrophoresis, and PstI ribotyping showed that endemic, epidemic multidrug-resistant typhoid fever was due, for at least 74.1% of the isolates, to one or two clones of serovar Typhi harboring a single resistance plasmid. PstI ribotyping was used as a basic technique to ensure that a serovar Typhi expansion was clonal.     

Prevalence and characterization of multidrug-resistant (type ACSSuT) Salmonella enterica serovar Typhimurium strains in isolates from four gosling farms and a hatchery farm

Salmonella enterica serovar Typhimurium strains of phage types DT104 and U302 are often resistant to ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline (the ACSSuT resistance type) and are major zoonotic pathogens. Increased consumption of goose meat may enhance the risk of transferring S. enterica serovar Typhimurium and other enteric pathogens from geese to human due to the consumption of meats from infected geese or improper preparation of meats. Therefore, we characterized S. enterica serovar Typhimurium strains isolated from four goose farms (farms A, B, C, and D) and one hatchery farm (farm E) to determine the epidemic and genetic differences among them. Antibiotic susceptibility tests and multiplex PCR confirmed that 77.6% (52/67) of strains were ACSSuT strains isolated from farms A, C, and E. Antibiotic-susceptible strains were isolated mostly from farm B, and no strain was observed in farm D. All ACSSuT strains harbored a 94.7-kb virulence plasmid and contained one 1.1-kb conserved segment identical to that of Salmonella genomic island 1. Four genotypes were determined among these S. enterica serovar Typhimurium isolates by pulsed-field gel electrophoresis analysis of XbaI-digested DNA fragments. Most isolates (85.29%; 29/34) of major genotype Ib were ACSSuT strains isolated mainly from goslings of farm C and egg membranes of farm E, a hatchery farm, suggesting that S. enterica serovar Typhimurium strains in isolates from goslings might originate from its hatchery, from the egg membranes to the gosling fluff after hatching. Multiple phage types, types 8, 12, U283, DT104, and U302, were identified. In conclusion, geese were a reservoir of diverse multidrug-resistant (type ACSSuT) S. enterica serovar Typhimurium strains, and each farm was colonized with genetically closely related S. enterica serovar Typhimurium strains.     

Occurrence of plasmids and antibiotic resistance among Campylobacter jejuni and Campylobacter coli isolated from healthy and diarrheic animals.

Serologically defined strains of Campylobacter jejuni and Campylobacter coli from healthy and diarrheic animals were examined for the occurrence of plasmid DNA in association with the antibiotic susceptibility of the bacterial host and the health status of the animal host. Of all campylobacter organisms surveyed, 53% (116 of 200) contained plasmid DNA. A plasmid occurrence rate of 73.8% was obtained for C. coli from healthy pigs, contrasted by lower plasmid occurrence rates for C. coli from diarrheic pigs (30%) and from all diarrheic animals (21.4%). For C. jejuni, in contrast, only 13.6% of healthy cattle contained plasmid DNA, contrasted by a higher plasmid occurrence rate of 31.2% from diarrheic cattle. A high plasmid occurrence rate of 75.8% was observed for C. jejuni from healthy chickens. Campylobacter plasmids ranged in size from less than or equal to 1 to 86 megadaltons. Antibiotic susceptibility for 52 animal isolates (excluding chickens) indicated that most isolates were susceptible to kanamycin, erythromycin, gentamicin, tetracycline, and compound sulfonamide, whereas few were susceptible to bacitracin (19.2%); approximately half were susceptible to ampicillin (55.8%) and streptomycin (51.9%), and no isolates were susceptible to penicillin G. More isolates containing plasmids were resistant to ampicillin, tetracycline, and gentamicin than were isolates not carrying plasmids, there being a statistically significant difference for tetracycline and gentamicin, which suggested that these two antibiotics were probably plasmid mediated. The antibiotic susceptibility patterns of 21 chicken isolates of C. jejuni, by contrast, were different in that most were susceptible to ampicillin in addition to kanamycin, erythromycin, and gentamicin, whereas few wer susceptible to compound sulfonamide, streptomycin, and tetracycline in addition to penicillin G and bacitracin. A 30- or 39-megadalton plasmid, or both, common to many of the chicken isolates was usually associated with tetracycline resistance.     

Activity of ampicillin in vitro compared with other antibiotics

Comparative tests in vitro for antibacterial activity were carried out with ampicillin, tetracycline, and chloramphenicol using 673 clinical isolates of Gram-negative bacilli and Streptococcus faecalis. Further comparative tests were also carried out with ampicillin, chloramphenicol, colistin sulphate, colistin methane sulphonate, cycloserine, kanamycin, nitrofurantoin, polymyxin, streptomycin, and tetracycline, using groups of 20 strains of each of the main species selected at random from the total number of isolates. Of the total number of isolates a higher percentage was inhibited by ampicillin than by tetracycline or chloramphenicol. Ampicillin showed particularly high activity against certain species of bacteria and displayed an antibacterial spectrum not shown by any of the other antibiotics tested.