Nucleotide sequence comparisons of plasmids pHD131, pJB1, pFA3, and pFA7 and beta-lactamase expression in Escherichia coli, Haemophilus influenzae, and Neisseria gonorrhoeae

There are three major Neisseria gonorrhoeae β-lactamase plasmid variants which are associated with specific lineages and carry distinct TEM alleles

Neisseria gonorrhoeae is a significant threat to global health with an estimated incidence of over 80 million cases each year and high levels of antimicrobial resistance. The gonococcal β-lactamase plasmid, pbla, carries the TEM β-lactamase, which requires only one or two amino acid changes to become an extended-spectrum β-lactamase (ESBL); this would render last resort treatments for gonorrhoea ineffective. Although pbla is not mobile, it can be transferred by the conjugative plasmid, pConj, found in N. gonorrhoeae. Seven variants of pbla have been described previously, but little is known about their frequency or distribution in the gonococcal population. We characterised sequences of pbla variants and devised a typing scheme, Ng_pblaST that allows their identification from whole genome short-read sequences. We implemented Ng_pblaST to assess the distribution of pbla variants in 15 532 gonococcal isolates. This demonstrated that only three pbla variants commonly circulate in gonococci, which together account for >99 % of sequences. The pbla variants carry different TEM alleles and are prevalent in distinct gonococcal lineages. Analysis of 2758 pbla-containing isolates revealed the co-occurrence of pbla with certain pConj types, indicating co-operativity between pbla and pConj variants in the spread of plasmid-mediated AMR in N. gonorrhoeae. Understanding the variation and distribution of pbla is essential for monitoring and predicting the spread of plasmid-mediated β-lactam resistance in N. gonorrhoeae.

Antibiotic Resistance Profiles of Haemophilus influenzae Isolates from Children in 2016: A Multicenter Study in China

BACKGROUND AND OBJECTIVE

Haemophilus influenzae (HI) is a common cause of community-acquired pneumonia in children. In many countries, HI strains are increasingly resistant to ampicillin and other commonly prescribed antibiotics, posing a challenge for effective clinical treatment. This study was undertaken to determine the antibiotic resistance profiles of HI isolates from Chinese children and to provide guidelines for clinical treatment.

METHODS

Our Infectious Disease Surveillance of Pediatrics (ISPED) collaboration group includes six children's hospitals in different regions of China. The same protocols and guidelines were used by all collaborators for the culture and identification of HI. The Kirby-Bauer method was used to test antibiotic susceptibility, and a cefinase disc was used to detect β-lactamase activity.

RESULTS

We isolated 2073 HI strains in 2016: 83.9% from the respiratory tract, 11.1% from vaginal secretions, and 0.5% from blood. Patients with respiratory isolates were significantly younger than nonrespiratory patients (P < 0.001). Of all 2073 strains, 50.3% were positive for β-lactamase and 58.1% were resistant to ampicillin; 9.3% were β-lactamase-negative and ampicillin-resistant. The resistance rates of the HI isolates to trimethoprim-sulfamethoxazole, azithromycin, cefuroxime, ampicillin-sulbactam, cefotaxime, and meropenem were 71.1%, 32.0%, 31.2%, 17.6%, 5.9%, and 0.2%, respectively.

CONCLUSIONS

More than half of the HI strains isolated from Chinese children were resistant to ampicillin, primarily due to the production of β-lactamase. Cefotaxime and other third-generation cephalosporins could be the first choice for the treatment of ampicillin-resistant HI infections.

Piperacillin-Tazobactam (TZP) Resistance in Escherichia coli Due to Hyperproduction of TEM-1 β-Lactamase Mediated by the Promoter Pa/Pb

TEM-1, mediated by plasmid and transposon, is the most commonly encountered β-lactamase in Gram-negative bacteria. Four different promoters upstream of bla_TEM-related genes have been identified: the weak P3 promoter, and the strong promoters Pa/Pb, P4, and P5. In this study, we investigated the genetic basis of a clinical strain of Escherichia coli (RJ904), which was found to be resistant to BLBLIs (β-lactam/β-lactamase inhibitors), including amoxicillin-clavulanate, ticarcillin-clavulanate (TCC), and piperacillin-tazobactam (TZP) but sensitive to third-generation cephalosporins. The conjugation test and S1-nuclease pulsed-field gel electrophoresis (S1-PFGE) demonstrated that transfer of this resistance was mediated by a ca. 100 kb plasmid. The transformant with TZP resistance was screened out with the shortgun cloning. Sequence analysis revealed that the recombinant plasmid contained a bla_TEM-1b gene with the strong promoter Pa/Pb. Different plasmids were cloned based on the clone vector pACYC184 with the insertion of the bla_TEM-1b gene with promoters Pa/Pb or P3. Susceptibility to TZP was determined by the E-test, agar dilution, and broth microdilution. The level of bla_TEM-1b-specific transcription was determined by quantitative real-time PCR. Substitution of Pa/Pb for P3 resulted in a 128-fold decline of the MIC value of TZP, from >1024 mg/L to 8 mg/L, and a significantly lower bla_TEM-1b expression level. Hyperproduction of TEM-1 β-lactamase mediated by the promoter Pa/Pb was responsible for high resistance to TZP in E. coli. Our data show possible risks of resistance development in association with the clinical use of TZP. The bla_TEM promoter modifications should be considered for whole genome whole-genome sequencing-inferred bacterial antimicrobial susceptibility testing.

The return of Pfeiffer's bacillus: Rising incidence of ampicillin resistance in Haemophilus influenzae

Haemophilus influenzae, originally named Pfeiffer's bacillus after its discoverer Richard Pfeiffer in 1892, was a major risk for global health at the beginning of the 20th century, causing childhood pneumonia and invasive disease as well as otitis media and other upper respiratory tract infections. The implementation of the Hib vaccine, targeting the major capsule type of H. influenzae, almost eradicated the disease in countries that adapted the vaccination scheme. However, a rising number of infections are caused by non-typeable H. influenzae (NTHi), which has no capsule and against which the vaccine therefore provides no protection, as well as other serotypes equally not recognised by the vaccine. The first line of treatment is ampicillin, but there is a steady rise in ampicillin resistance. This is both through acquired as well as intrinsic mechanisms, and is cause for serious concern and the need for more surveillance. There are also increasing reports of new modifications of the intrinsic ampicillin-resistance mechanism leading to resistance against cephalosporins and carbapenems, the last line of well-tolerated drugs, and ampicillin-resistant H. influenzae was included in the recently released priority list of antibiotic-resistant bacteria by the WHO. This review provides an overview of ampicillin resistance prevalence and mechanisms in the context of our current knowledge about population dynamics of H. influenzae.

Antimicrobial resistance in Haemophilus influenzae

Haemophilus influenzae is a major community-acquired pathogen causing significant morbidity and mortality worldwide. Meningitis and bacteremia due to type b strains occur in areas where the protein-conjugated type b vaccine is not in use, whereas nontypeable strains are major causes of otitis media, sinusitis, acute exacerbations of chronic bronchitis, and pneumonia. Antibiotic resistance in this organism is more diverse and widespread than is commonly appreciated. Intrinsic efflux resistance mechanisms limit the activity of the macrolides, azalides, and ketolides. beta-Lactamase production is highly prevalent worldwide and is associated with resistance to ampicillin and amoxicillin. Strains with alterations in penicillin binding proteins, particularly PBP3 (beta-lactamase negative ampicillin resistant and beta-lactamase positive amoxicillin-clavulanate resistant), are increasing in prevalence, particularly in Japan, with increasing resistance to ampicillin, amoxicillin, amoxicillin-clavulanate, and many cephalosporins, limiting the efficacy of expanded-spectrum cephalosporins against meningitis and of many oral cephalosporins against other diseases. Most strains remain susceptible to the carbapenems, which are not affected by penicillin binding protein changes, and the quinolones. The activity of many oral agents is limited by pharmacokinetics achieved with administration by this route, and the susceptibility of isolates based on pharmacokinetic and pharmacodynamic parameters is reviewed.

Variation in selected regions of blaTEM genes and promoters in Haemophilus influenzae

OBJECTIVES

To determine if the TEM beta-lactamases of Haemophilus influenzae are TEM-1 or derivatives thereof and associated exclusively with the overlapping Pa/Pb promoters.

METHODS

Single nucleotide specific PCR was used to discriminate the polymorphic nucleotides at positions 32 and 317 of the bla(TEM) genes of a collection of TEM-positive strains.

RESULTS

All bla(TEM) genes were found to be bla(TEM-1) or derivatives thereof and none bla(TEM-2). The bla(TEM) genes were associated with the P3 promoter, the Pa/Pb promoters or a novel promoter produced as a result of a 135 bp deletion and a G162T substitution.

CONCLUSIONS

The genetic features of bla(TEM) genes in H. influenzae are different from those in Enterobacteriaceae and more variable than previously recognized.

Multiple origins and replication proteins influence biological properties of beta-lactamase-producing plasmids from Neisseria gonorrhoeae

The beta-lactamase-producing Asia-type plasmid pJD4 of Neisseria gonorrhoeae is a 7.4-kb, broad-host-range plasmid. It is part of a family of plasmids which are structurally related yet vary in size, found in both N. gonorrhoeae and Haemophilus ducreyi. Branch-point analysis by electron microscopy indicates that pJD4 carries three clustered but distinguishable origins of replication, which we named ori1, ori2, and ori3. Although pJD4 belongs to incompatibility (Inc) group W, it also carries a silent IncFII determinant which is expressed when ori2 and ori3 are absent. The Africa-type plasmid pJD5, a naturally occurring deletion derivative of pJD4, carries only ori1, belongs to the IncFII group, and, in contrast to pJD4, requires DNA polymerase I (Pol I) for replication. Plasmids constructed from pJD4 which lack ori1 but carry ori2 and ori3 do not require Pol I and are incompatible with IncW plasmids, suggesting that the ori2 or ori3 region contains the IncW determinant. We have cloned a replication initiation protein (RepB) that is necessary for ori2 and ori3 to function. This Rep protein is distinct from RepA, which is necessary for ori1. Thus, pJD4 is unique because it is the smallest plasmid characterized containing three origins of replication and two unique Rep proteins.

Antimicrobial drug use and resistance among respiratory pathogens in the community

There is substantial evidence that the overuse of antibiotics is a major cause for the emergence of resistance in respiratory pathogens in the community. However, it is also recognized that the mechanisms of resistance, the cost of resistance to the fitness of the organism, and the ability of the resistant strain to disseminate are all important contributors to this problem. Therefore, when developing strategies to control and/or prevent the emergence of resistance, health care professionals must take each of these factors into consideration. As we enter a new era in the use of fluoroquinolones for the treatment of respiratory tract infections, we have an opportunity to apply such lessons learned in the past to minimize or prevent the development of resistance to this class of antimicrobial drugs in the future.

Complete sequence of a beta-lactamase-encoding plasmid in Neisseria meningitidis

Identical beta-lactamase-encoding (TEM-1) plasmids were found in two different clinical Neisseria meningitidis strains. They were completely sequenced (5,597 bp) and designated pAB6. The plasmid is almost identical to Neisseria gonorrhoeae plasmid pJD5 (5,599 kb) and may have been picked up from a gonococcus in vivo.

Sequence analysis of the family of penicillinase-producing plasmids of Neisseria gonorrhoeae

The exact nature of the sequence differences between the medically important family of gonococcal penicillinase-producing plasmids has been ascertained. The entire DNA sequence of the Asia-type plasmid, pJD4, demonstrated that it is 7426 bp and contains two direct repeats (DR30) that are implicated in the formation of deletion variant plasmids, such as the Africa-type plasmid. We have identified putative DnaA and IHF binding sites, various open reading frames that are thought to specify functional proteins, and some important DNA sequences involved with conjugative transfer of gonococcal beta-lactamase plasmids. The deletion in the Africa-type plasmid is 1827 bp and one of the DR30 repeats is also missing. The deletion in the Rio-type plasmid and several Toronto-type plasmids was determined to be 2273 bp and the sequence spanning the deletion was identical irrespective of geographic or temporal origin. The &Ncirc;imes-type plasmid is an Africa-type plasmid and also contains an IS5 insertion sequence. Since IS5 has not been identified in gonococcal isolates, we suggest that this sequence may have been inserted after the original gonococcal plasmid was transformed into Escherichia coli. The New Zealand plasmid is an Asia-type plasmid that contains an endogenous tandem duplication of 1883 bp and the direct DR2 is implicated in this duplication. The nature of the defined truncation of Tn2 present in the various plasmids is also discussed.

Antibiotic resistance in oral/respiratory bacteria

In the last 20 years, changes in world technology have occurred which have allowed for the rapid transport of people, food, and goods. Unfortunately, antibiotic residues and antibiotic-resistant bacteria have been transported as well. Over the past 20 years, the rise in antibiotic-resistant gene carriage in virtually every species of bacteria, not just oral/respiratory bacteria, has been documented. In this review, the main mechanisms of resistance to the important antibiotics used for treatment of disease caused by oral/respiratory bacteria--including beta-lactams, tetracycline, and metronidazole--are discussed in detail. Mechanisms of resistance for macrolides, lincosamides, streptogramins, trimethoprim, sulfonamides, aminoglycosides, and chloramphenicol are also discussed, along with the possible role that mercury resistance may play in the bacterial ecology.

Antimicrobial resistance of Neisseria gonorrhoeae in Liberia

The prevalence and molecular characteristics of penicillinase-producing Neisseria gonorrhoeae (PPNG) and tetracycline-resistant N. gonorrhoeae (TRNG) were determined in 10 clinics in Monrovia, Liberia, to assess the likely effectiveness of the current standard treatment with penicillin or tetracycline. One hundred gonococcal strains were isolated from 146 urethral swabs and 261 cervical swabs and screened for susceptibility to ceftriaxone, penicillin, spectinomycin and tetracycline by the disk diffusion method; 83% were resistant to penicillin and 63% to tetracycline. Twenty-one strains from 18 men and 3 women with uncomplicated gonorrhoea were subjected to more detailed characterization. These 21 strains belonged to 5 auxotype/serovar classes; 86% were PPNG/TRNG. Three PPNG harboured the 4.4 MDa penicillinase plasmid and 16 the 3.2 MDa plasmid. All TRNG harboured the 25.2 MDa plasmid and their MICs for tetracycline were > 32 mg/L. They gave a PCR product which, according to its restriction pattern, corresponded to the American type tetM gene. By the agar dilution method, all strains exhibited intermediate resistance to sulphamethoxazole-trimethoprim (19:1) (co-trimoxazole) with MICs of 8-32 mg/L. All strains were susceptible to spectinomycin and ciprofloxacin. The MICs for gentamicin were 4-8 mg/L. The use of effective and affordable antimicrobial chemotherapy with either 500 mg ciprofloxacin or a single dose of gentamicin is discussed, with consideration of molecular biological, pharmacological and public health aspects.

Identification of a streptomycin resistance gene and a partial Tn3 transposon coding for a beta-lactamase in a periodontal strain of Eikenella corrodens

The beta-lactamase gene from a periodontal strain of Eikenella corrodens, resistant to penicillins and streptomycin, was inserted into pBGS9 and transformed into Escherichia coli DH5 alpha. A 4.7-kb insert of pJML1, one of the transformants, was partially sequenced and found to contain the right section of transposon Tn3 from the middle of the TnpR resolvase gene to the right inverted repeat RI(R), including the TEM-1 gene. Sequences identical to RSF1010 were found on either side of the Tn3 sequence. pJML1 also contained a streptomycin resistance gene, probably identical to that of RSF1010. A portion of the pJML1 insert was not homologous to either Tn3 or RSF1010 but was homologous to the chromosomal DNA of E. corrodens ATCC 23834. It is assumed that the insert of pJML1 was derived from the chromosomal DNA of E. corrodens EC-38.

Sequence and molecular characterization of the ROB-1 beta-lactamase gene from Pasteurella haemolytica

The ROB-1 beta-lactamase-encoding plasmids from eight Pasteurella and two Haemophilus strains were compared by restriction endonuclease and hybridization analyses. Two types of ROB-1-encoding plasmids, which differed in size, were detected. One (4.1 kb) was found only in Pasteurella strains. The other (4.4 kb) was found in both Haemophilus influenzae and in one of the eight Pasteurella strains examined. These two plasmids shared multiple homologous fragments, suggesting that one was derived from the other. The ROB-1-encoding gene from Pasteurella haemolytica LNPB 51 was cloned and sequenced. An open reading frame of 915 nucleotides was found; it encoded a 305-amino-acid protein. Analysis of this amino acid sequence confirmed that the enzyme was found; it encoded a 305-amino-acid protein. Analysis of this amino acid sequence confirmed that the enzyme is a class A beta-lactamase. It had 32 to 48% homology with other class A enzymes and exhibited several common features of the gram-positive beta-lactamases. The ROB-1 mature protein, however, contained only one cysteine residue at position 123. These results suggest that ROB-1 is a link between beta-lactamases of gram-negative and gram-positive bacteria. An internal 230-bp DraI fragment from ROB-1 hybridized only with plasmid DNA from ROB-1-producing strains. This specific probe could be useful in epidemiological studies.

TnA transposons can be introduced and maintained in Neisseria gonorrhoeae

In Neisseria gonorrhoeae, all penicillinase-specifying plasmids isolated so far share homology with each other and carry a 60% deleted sequence of TnA. Plasmid pHD131, an element isolated from Haemophilus ducreyi and carrying an intact copy of the ampicillin resistance transposable element, was introduced from Escherichia coli into N. gonorrhoeae by both transformation and conjugative mobilization. Plasmids were recovered with no detectable deletion. After their transfer back into E. coli, transposition assays onto phage-lambda DNA demonstrated that the TnA elements were still functional. Plasmid pHD131 could be stably maintained in N. gonorrhoeae with or without the presence of penicillin. These results support the hypothesis that the absence in N. gonorrhoeae of plasmids carrying entire and functional TnA transposons cannot be ascribed to incompatibility between the genetic element and the host, but rather to a barrier to introduction of foreign DNA into gonococcus.

Physiological effect of the alterations of plasmids derived from Neisseria gonorrhoeae

Transfer of plasmid DNA from penicillinase-producing Neisseria gonorrhoeae to Escherichia coli produced relatively small colonies that became pigmented in the presence of the dye, congo red. Mutations to large, non-pigmented colonies were frequent, and were the result of a variety of alterations in these plasmids. Miniplasmid formation was the major alteration that led to non-pigmented colonies. However, deletions in any of three putative replication regions resulted in non-pigmented colonies. These altered plasmids mediated significantly less beta-lactamase production than that of the parent plasmid. A new origin of replication was discovered and mapped within the miniplasmid. This origin was responsible for: 1) high plasmid copy number, 2) stability of inheritance, and 3) the majority of beta-lactamase produced in culture. The evolution of gonococcal plasmids may be followed in E. coli, since less defective plasmids produce nonpigmented colonies on congo red agar medium.

Identification and characterization of a new replication region in the Neisseria gonorrhoeae beta-lactamase plasmid pFA3

The 7.1-kilobase-pair (kbp) plasmid pFA3 specifies TEM beta-lactamase production in Neisseria gonorrhoeae. We studied the minimal region required for replication of this plasmid in Escherichia coli by constructing a set of nested deletions of the 3.4-kbp PstI-HindIII fragment. The smallest fragment capable of maintenance in E. coli when ligated to a streptomycin-spectinomycin resistance cassette was 2.0 kbp in size and was different from another autonomously replicating fragment of pFA3 reported by K. H. Yeung and J. Dillon (Plasmid 20:232-240, 1988). The fragment contained single BamHI and XbaI sites and specified a 39-K protein. Fragments subcloned from the minimal region or constructed by deletion from the 3' or 5' ends were not capable of autonomous replication. Mutants constructed by end filling and religating DNA cleaved at the BamHI or XbaI sites were not capable of autonomous replication and no longer produced the 39K protein. These results suggest that replication is dependent on the 39K protein. DNA sequence analysis of the region showed an A-T-rich region followed by four 22-bp direct repeats followed by an open reading frame encoding a 39K basic protein.

Characterization of the Tet M determinants in urogenital and respiratory bacteria

Tetracycline-resistant Fusobacterium nucleatum, Haemophilus ducreyi, Mycoplasma hominis, Peptostreptococcus spp., Ureaplasma urealyticum, and Veillonella parvula had DNA sequences which showed homology throughout the length of the Tet M transposon, Tn916. In contrast, Gardnerella vaginalis, commensal Neisseria spp., and the 25.2-megadalton plasmid family lacked the complete transposon.

Biology of Haemophilus ducreyi

The etiological agent of the sexually transmitted genital ulcer disease chancroid was first described in 1889 by Auguste Ducrey following repeated autoinoculation of purulent ulcer material from a series of patients. The organism was isolated on artificial media a decade later but has remained difficult to isolate consistently, resulting in controversy over its characteristics and role as the causative agent of chancroid. Because of its fastidious growth requirements, including unknown components in blood, the organism was included in the original description of the genus Haemophilus. Requirement for exogenous hemin and limited phenotypic characteristics, including structural and antigenic properties, suggested that Haemophilus ducreyi was a valid member of the genus Haemophilus. Recent studies of respiratory quinones, deoxyribonucleic acid hybridization, and competition for homologous transformation of the type species, H. influenzae, suggest that H. ducreyi is unrelated to any of the present species of the family Pasteurellaceae, which includes members of the genera Haemophilus, Actinobacillus, and Pasteurella. This review summarizes the early studies with H. ducreyi and our current knowledge of the microbiology of this important human pathogen.

Export and intercellular transfer of DNA via membrane blebs of Neisseria gonorrhoeae

Naturally elaborated membrane bleb material is frequently observed in cultures of Neisseria gonorrhoeae. This material was purified and analyzed for protein, lipopolysaccharide, and nucleic acid content. The electrophoretic protein profiles of two bleb-rich fractions, called BI and BII, were distinct, with only BII containing lipopolysaccharide and outer membrane proteins I and III. Both fractions contained RNA, circular DNA, and linear DNA. Exogenous pancreatic DNase I appeared to hydrolyze all bleb-associated DNA in fraction BI and the linear DNA in fraction BII. The circular DNA molecules associated with fraction BII resisted digestion. Electron microscopy of the bleb fractions verified their DNA content. Fixing blebs with glutaraldehyde before mounting them for microscopy prevented release of internal DNA. Such fixation produced little change in the micrographs of BI; however, only traces of DNA were observed in fixed BII preparations. Incubation of wild-type gonococci in mixtures of DNase and blebs purified from antibiotic-resistant strains resulted in efficient exchange of penicillinase-specifying R plasmids. Recipients incorporated plasmids independently of endogenous and exogenous chromosomal streptomycin resistance markers. These in vitro results suggest that bleb formation by N. gonorrhoeae may serve to transfer plasmids intercellularly in vivo, perhaps constituting a previously unexplored genetic exchange mechanism in these bacteria.

Beta-lactamase plasmids and chromosomally mediated antibiotic resistance in pathogenic Neisseria species

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Chancroid and Haemophilus ducreyi

Haemophilus ducreyi is the causative agent of chancroid, one of the genital ulcerative diseases. H. ducreyi is the major cause of genital ulcer disease in Africa and Southeast Asia and is of increasing concern in the United States. Definitive diagnosis of chancroid requires the isolation and identification of H. ducreyi, but isolation of this organism is difficult and the available medium is not optimal for all strains. Fluorescent antibody and serologic tests are of limited value. In general, our knowledge of this organism is rather limited, and indeed, recent studies have questioned the placement of H. ducreyi in the genus Haemophilus. H. ducreyi has relatively few biochemical activities, and epidemiologic studies are limited because there are limited phenotypic markers available for strain typing. Specific virulence factors of H. ducreyi have yet to be identified. Antimicrobial resistance in H. ducreyi is of special concern, as this organism has acquired both gram-negative and gram-positive resistance determinants. In addition, some of these determinants can be mobilized and transferred to other Haemophilus species or to Neisseria gonorrhoeae.

Rapid chemiluminescent nucleic acid assays for detection of TEM-1 beta-lactamase-mediated penicillin resistance in Neisseria gonorrhoeae and other bacteria

Two new assays for the detection of TEM-1 beta-lactamase-mediated bacterial penicillin resistance were developed that involve the use of specific nucleic acid hybridization. Both techniques are based on a solution-phase hybridization of oligonucleotide probes to the target DNA sequence, solid-phase capture of the probe-target complex, and an amplified chemiluminescent labeling method. One configuration of hybridization probes detected the presence of TEM-1 in Neisseria gonorrhoeae (45 strains), Haemophilus spp., Escherichia coli, Shigella sonnei and Salmonella typhi. A second configuration (TEM-1NH) detected TEM-1 beta-lactamase-mediated penicillin resistance only in N. gonorrhoeae (97 strains) and Haemophilus (6 strains) isolates in which TEM-1 is inserted in a pFA7-type plasmid. Both methods were 100 times more sensitive than a commercially available colorimetric beta-lactamase activity test and approximately 5 times more sensitive than radioisotopic dot blot screening for the gene. The assays are particularly well suited to the analysis of large numbers of samples, can be performed in a total of 4 h, and are sensitive to 10(4) to 10(5) CFU.

Genetic determinant of the ROB-1 beta-lactamase in bovine and porcine Pasteurella strains

The ROB-1 beta-lactamase, previously described in Haemophilus influenzae, has been found in the genus Pasteurella. In three bovine strains of Pasteurella multocida and Pasteurella haemolytica, ROB-1 production was determined by plasmids of 4.4 kilobases. In one porcine strain of Pasteurella aerogenes, the enzyme seems to be chromosomally encoded.