Virulence genes of bovine Staphylococcus aureus from persistent and nonpersistent intramammary infections with different clinical characteristics

AIMS

To screen putative virulence genes in Staphylococcus aureus causing persistent and nonpersistent bovine intramammary infections (IMI) with different clinical characteristics. To examine, whether a possible relationship exists between genetic profile and infection persistence, clinical signs of infection, clonal type determined by pulsed-field gel electrophoresis (PFGE), and antimicrobial resistance.

METHODS AND RESULTS

One-hundred and sixty-one S. aureus isolates derived from bovine IMI, consisting of 17 different PFGE types, were screened by conventional and multiplex-polymerase chain reaction (PCR) for 24 virulence genes for haemolysins (hla-hlg), leukocidins (lukED, lukM), exfoliative toxins (eta, etb), enterotoxins (sea-seo, seu), toxic-shock syndrome toxin (tst), and genes encoding penicillin (blaZ) and methicillin resistance (mecA). The majority of S. aureus isolated at the onset of mastitis carried haemolysin genes (76.7-97.4%), lukED (96.6%), and at least one gene for pyrogenic toxin superantigen (PTSAg) (69.0%). Strains carrying PTSAg-encoding genes were more common among predominant PFGE types and in persistent IMI. Strains concomitantly possessing sed, sej, and blaZ, putatively plasmid-encoded, were typically found in connection with persistent IMI.

CONCLUSIONS

Our results suggest that certain genetic elements are over-representative in S. aureus isolates especially from persistent bovine mastitis. This phenomenon seems to be in connection with clonal type and is often concomitant with penicillin resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study to investigate associations between a large number of bacterial factors and outcome of S. aureus mastitis. The finding that widespread clonal types of S. aureus causing bovine mastitis of low treatment response may harbour characteristic genes could be improved for strain-specific diagnostic purposes.

Bacterial genotype affects the manifestation and persistence of bovine Staphylococcus aureus intramammary infection

Two-hundred seventeen Staphylococcus aureus isolates from 116 dairy cows with intramammary infections were analyzed by pulsed-field gel electrophoresis to study the association between symptom severity, persistence of infection, and bacterial genotype. Among five main genotypes infecting 90% of the cows, one was associated with severe clinical symptoms but reduced persistence.

Comparison of phenotypic and genotypic detection of penicillin G resistance of Staphylococcus aureus isolated from bovine intramammary infection

Resistance of Staphylococcus aureus to penicillin G is common among isolates from bovine mastitis. We determined phenotypic resistance to penicillin G for 151 S. aureus isolates derived from dairy cows with intramammary infection by two methods. The methods were determination of minimum inhibitory concentration (MIC) by a standard agar dilution technique and direct testing of beta-lactamase production using a chromogenic cephalosporin, nitrocefin. The results from these tests were compared with the presence of the beta-lactamase (blaZ) gene in the isolates, which was detected by polymerase chain reaction (PCR). Testing beta-lactamase production with nitrocefin was more predictive for the presence of the blaZ gene than the agar dilution method and the results of the former agreed highly with the presence of the blaZ gene in the isolates. In contrast, the resistance breakpoint generally used in the agar dilution method may be too high for prediction of penicillin resistance in S. aureus isolates with borderline MICs. Using this method, 40% of the isolates possessing the blaZ gene were classified as susceptible; however, majority of these isolates produced beta-lactamase when tested with nitrocefin.

Bovine Mastitis in Finland 2001—Prevalence, Distribution of Bacteria, and Antimicrobial Resistance

A nationwide survey was conducted in Finland to estimate prevalence of bovine mastitis, distribution of mastitis pathogens, and in vitro antimicrobial susceptibility of different mastitis pathogens. In total, 12,661 quarter milk samples were collected from 3282 dairy cows at 216 farms. These were randomly selected from a database covering all Finnish dairy farms. Quarter milk samples collected by the dairy advisors were submitted for somatic cell counting, bacteriological examination, and testing for antimicrobial susceptibility. If the milk SCC of a cow or of a quarter exceeded 300,000/mL, the cow was defined as having mastitis. The results were compared with those of a previous survey done in 1995. The prevalence of mastitis continued to decrease from 38% in 1995 to 31% in 2001. Compared with the study from 1995, the number of quarters with bacterial growth in 2001 increased significantly from 21.0 to 33.5%. This mainly resulted from increased prevalence of Corynebacterium bovis. Coagulase-negative staphylococci remained the most common bacterial group, comprising almost one-half of the pathogens isolated, whereas the relative number of Staphylococcus aureus isolations decreased from the time of the previous study. According to in vitro antimicrobial susceptibility testing, the enterococci demonstrated the highest level of resistance. Compared with the other Nordic countries, penicillin resistance among the staphylococci was still at a relatively high level in Finland (52.1 and 32.0% for Staphylococcus aureus and coagulase-negative staphylococci, respectively). Streptococci isolated from mastitis were very susceptible to beta-lactam antibiotics, as also found in the previous survey in 1995.

Multiplanar and 3D CT of acetabular fractures

PURPOSE

To establish a standard protocol for the multiplanar (MPR) and 3D shaded surface display (SSD) reconstruction of CT data on acetabular fractures, and to assess the usefulness of these reformats.

MATERIAL AND METHODS

Acetabular fractures in 15 patients were imaged by means of plain radiographs, transaxial CT, MPR reformats, and SSD reformats.

RESULTS

The classification of the acetabular fracture was revised in 7/15 cases when the transaxial CT images were read after the plain radiographs. Although the MPR and SSD reformats did not alter the classification, they did add to the degree of confidence in the diagnosis in 9/15 cases. In 2 patients, the MPR and SSD reformats indicated operative instead of conservative treatment. In the MPR reformats, the following views were considered essential in all cases: (a) along the anterior column; (b) along the posterior column; and (c) along both columns and the inferior ramus. In the SSD reformats, the following views were considered essential in all cases: (d) the latero-caudal en face view into the acetabulum; and 180 degrees opposite to this, (e) the medio-cranial view (facing the quadrilateral plate). In 10/15 cases, these views were all that was needed for classification. It was, however, essential to remove the femur from the images before reconstructing the SSD views.

CONCLUSION

Complex acetabular fractures with displacement should be evaluated by means of transaxial CT and additional MPR and SSD reformats. The use of appropriate standard MPR and SSD views shortens the time required to produce the reformats and thereby maximizes the benefit gained.

A simple device for the stereoscopic display of 3D CT images

We describe a simple device for creating true 3D views of image pairs obtained at 3D CT reconstruction. The device presents the images in a slightly different angle of view for the left and the right eyes. This true 3D viewing technique was applied experimentally in the evaluation of complex acetabular fractures. Experiments were also made to determine the optimal angle between the images for each eye. The angle varied between 1 degree and 7 degrees for different observers and also depended on the display field of view used.

The value of routine sonography in clinically suspected acute cholecystitis

Upper abdominal sonography was used as a routine emergency study to diagnose acute cholecystitis in 135 patients clinically suspected of having the disease. Ten radiologists with various experience in sonography performed the studies. Fifty-six patients had acute cholecystitis. Altogether 52 cholecystectomies were performed, mainly within 48 h of admission. Acute cholecystitis was diagnosed correctly in 52 cases (sensitivity, 93%) and excluded correctly in 75 cases (specificity, 95%; overall accuracy, 94%). Of the four patients with a false-negative study, calculi without signs of cholecystitis were detected in three, and distention and tenderness without calculi in one case. The final diagnoses in four false-positive studies were chronic cholecystitis in two cases, carcinoma of the gallbladder in one case, and pancreatitis in one case. The results of sonography as a continual emergency service provided by a staff with various experience are equal to those published in other studies performed mainly by an expert staff with long experience.