- Volume 49, Issue 12, 2000

The pathogenesis of pneumococcal infection is a complex interplay between pneumococcal virulence determinants and the host immune response. Molecular studies have considerably advanced our knowledge and understanding of the precise structures and functions of the different determinants and their pathogenic roles. This review describes the mechanisms by which pneumococci attach, invade, evade lung defences and cause severe disease. Better understanding of the critical steps in this complex process will enable more effective clinical intervention to be developed to reduce the mortality exacted by this versatile pathogen.

The presence of class 1 integrons in clinical isolates of Escherichia coli was detected by PCR. Of 104 E. coli isolates from Kaohsiung, 54 (52%) carried class 1 integrons, with inserted DNA regions of 1–3 kb. These integrons were located on plasmids, as demonstrated by Southern hybridisation. DNA sequencing was used to identify the genetic content of the integron-variable regions. Different class 1 integrons contained various numbers, kinds and combinations of gene cassettes within their variable regions. These gene cassettes included those encoding resistance to trimethoprim (dfrIa, dfrV, dfr12 and dfr17), aminoglycosides (aadA1a, aadA2, aadA4 and aadB), chloramphenicol (cmlA), erythromycin (ereA2) and β-lactams (blaP1). An integron carrying three inserted cassettes – dfr12-orfF-aadA2 – was present in 33 (61%) of the 54 isolates with class 1 integrons. Gene cassettes encoding resistance were expressed phenotypically. The results indicate that class 1 integrons are widespread in clinical E. coli isolates in Taiwan. The types, combinations and frequency of the gene cassettes in integrons may reflect the specific selective pressures to which the isolates were exposed and could provide useful surveillance data for relation to antibiotic usage information.

Nosocomial infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a major cause of outbreaks in intensive care units. Infants make up a sector of the population that presents a high risk for MRSA infections. Mother-to-infant transmission has been indicated as a possible cause of MRSA infections in neonates. The occurrence and characteristics of MRSA in samples of banked human milk were investigated by selective culture, antibiogram and pulsed-field gel electrophoresis. MRSA contamination was found in 11% of 500 samples of expressed, fresh-frozen milk from 500 different donors at five Brazilian milk banks. The great majority of the contaminated samples passed breast milk quality control criteria for dispensing as raw milk under Brazilian and American guidelines. Most of the MRSA isolates belonged to the Brazilian epidemic clone, which is reported to be widespread in several Brazilian states, in Argentina and in Portugal. It is concluded that expressed breast milk can be a reservoir of multiresistant S. aureus epidemic clones. Studies are necessary to assess the source of contamination and potential role of MRSA-contaminated milk in the transmission of MRSA to neonates.

Oligonucleotide primers were designed based upon a comparison of the dextranase gene (dex) sequences from Streptococcus sobrinus and S. mutans. The primers amplified a 1610-bp long DNA fragment on the dex gene by a PCR. The pair of primers was specific to S. sobrinus as the other members of the mutans streptococci – S. mutans, S. downei, S. cricetus, S. rattus, S. macacae and S. ferus – gave no PCR products. Other gram-positive oral bacteria (15 strains of 10 species of cocci and 18 strains of 12 species of rods) and gram-negative oral bacteria (3 strains of 3 species of cocci and 31 strains of 22 species of rods) also gave negative results in the PCR. The PCR procedure was able to detect as little as 100 fg of purified chromosomal DNA or as few as 9 cfu of S. sobrinus NIDR6715. Seven clinical isolates of S. sobrinus were also positive in the dex PCR. This laboratory developed the S. mutans-specific PCR (dexA PCR) method with the primers specific for a portion of the dextranase gene of S. mutans Ingbritt. Primers for the dex and dexA PCR methods detected two species exclusively from the mutans streptococci. Furthermore, these two species were effectively differentiated by the species-specific amplicons with different lengths. The application of the PCR method to human dental plaque showed that the prevalence of S. sobrinus (83%) in oral cavities was higher than currently supposed (0–50%). These results suggest that the described PCR method is suitable for the specific detection and identification of human cariogenic bacteria, S. sobrinus and S. mutans.

A rise in the incidence of meningococcal disease has occurred in Spain in recent years, especially in some regions in the north-west of the country. Most cases have been caused by meningococci characterised as Neisseria meningitidis C:2b:P1.2,5. A total of 107 C:2b:P1.2,5 meningococcal isolates (60 from patients and 47 from carriers) and 12 isolates showing related antigenic combinations (C:2b:NST, C:2b:P1.2, C:2b:P1.5, C:NT:P1.2,5) was analysed by pulsed-field gel electrophoresis to determine the genetic variability of the epidemic and related strains. Endonucleases BglII and NheI were used to cut chromosomal DNA. When BglII was used, most of the C:2b:P1.2,5 isolates showed the same pulsotype regardless of whether they were from clinical cases or carriers. Isolates showing the principal profile after digestion with endonuclease BglII were analysed with NheI. Four pulsotypes were identified, of which two were found in only one isolate each. The major profiles (1 and 2) showed differential distribution among clinical and carrier isolates; pulsotype 1 was the most frequent among clinical isolates. However, the proportions of isolates showing profiles 1 and 2 were similar among carrier isolates. This could indicate that there are two variants of the C:2b:P1.2,5 strain with differing pathogenicity.

Numerous respiratory bovine coronaviruses (RBCV) were isolated recently from nasal swab samples and lung tissues of feedlot cattle with acute respiratory tract disease. These newly emerging RBCV isolates exhibited distinct phenotypic features that differentiated them from enteropathogenic bovine coronaviruses (EBCV). The RBCV strains had a receptor-destroying enzyme function mediated by acetylesterase (AE) activity of the haemagglutinin-esterase (HE) glycoprotein. The HE genes of wild-type EBCV strain LY138 and RBCV strains OK-0514 (OK) and LSU-94LSS-051 (LSU) were cloned, sequenced and transiently expressed in COS-7 cells. The enzymic properties of HE proteins in COS-7 cellular extracts and in purified virus preparations were assayed at room temperature, 37°C and 39°C by two different assays. One assay used ρ-nitrophenyl acetate (PNPA) as substrate and detected serine-esterase activity; the second assay monitored AE function with bovine submaxillary mucin (BSM) as substrate. The PNPA tests confirmed that HE proteins of EBCV and RBCV were functionally expressed in transfected COS-7 cells. Time-dependent determination of the AE activity of purified RBCV OK and LSU particles showed lower AE activity at 39°C than at 37°C, whereas the purified EBCV LY particles retained full AE activity at both 37°C and 39°C. Transiently expressed RBCV HE exhibited a marked reduction of AE activity after 40 min of assay time at 37°C. In contrast, the AE activity of the transiently expressed EBCV HE remained stable beyond 40 min. The deduced amino-acid sequences of the HE proteins specified by the RBCV strains OK and LSU contained specific amino-acid changes in comparison with the EBCV LY strain, which may be responsible for the observed enzymic differences. These results are consistent with the hypothesis that RBCV strains have evolved to selectively replicate in respiratory tissues and that HE may play a role in this tissue tropism.