Respiratory deficient Staphylococcus aureus as the aetiological agent of "abscess disease"

Virulence Mechanisms of Staphylococcal Animal Pathogens

Staphylococci are major causes of infections in mammals. Mammals are colonized by diverse staphylococcal species, often with moderate to strong host specificity, and colonization is a common source of infection. Staphylococcal infections of animals not only are of major importance for animal well-being but have considerable economic consequences, such as in the case of staphylococcal mastitis, which costs billions of dollars annually. Furthermore, pet animals can be temporary carriers of strains infectious to humans. Moreover, antimicrobial resistance is a great concern in livestock infections, as there is considerable antibiotic overuse, and resistant strains can be transferred to humans. With the number of working antibiotics continuously becoming smaller due to the concomitant spread of resistant strains, alternative approaches, such as anti-virulence, are increasingly being investigated to treat staphylococcal infections. For this, understanding the virulence mechanisms of animal staphylococcal pathogens is crucial. While many virulence factors have similar functions in humans as animals, there are increasingly frequent reports of host-specific virulence factors and mechanisms. Furthermore, we are only beginning to understand virulence mechanisms in animal-specific staphylococcal pathogens. This review gives an overview of animal infections caused by staphylococci and our knowledge about the virulence mechanisms involved.

Radical genome remodelling accompanied the emergence of a novel host-restricted bacterial pathogen

The emergence of new pathogens is a major threat to public and veterinary health. Changes in bacterial habitat such as a switch in host or disease tropism are typically accompanied by genetic diversification. Staphylococcus aureus is a multi-host bacterial species associated with human and livestock infections. A microaerophilic subspecies, Staphylococcus aureus subsp. anaerobius, is responsible for Morel’s disease, a lymphadenitis restricted to sheep and goats. However, the evolutionary history of S. aureus subsp. anaerobius and its relatedness to S. aureus are unknown. Population genomic analyses of clinical S. aureus subsp. anaerobius isolates revealed a highly conserved clone that descended from a S. aureus progenitor about 1000 years ago before differentiating into distinct lineages that contain African and European isolates. S. aureus subsp. anaerobius has undergone limited clonal expansion, with a restricted population size, and an evolutionary rate 10-fold slower than S. aureus. The transition to its current restricted ecological niche involved acquisition of a pathogenicity island encoding a ruminant host-specific effector of abscess formation, large chromosomal re-arrangements, and the accumulation of at least 205 pseudogenes, resulting in a highly fastidious metabolism. Importantly, expansion of ~87 insertion sequences (IS) located largely in intergenic regions provided distinct mechanisms for the control of expression of flanking genes, including a novel mechanism associated with IS-mediated anti-anti-sense decoupling of ancestral gene repression. Our findings reveal the remarkable evolutionary trajectory of a host-restricted bacterial pathogen that resulted from extensive remodelling of the S. aureus genome through an array of diverse mechanisms in parallel.

The emergence of new pathogens is a major threat to public and veterinary health. Some bacteria such as Staphylococcus aureus, have the capacity to infect many different host species including humans and livestock while others such as the closely-related S. aureus subsp. anaerobius, associated with a single type of pathology called Morel’s disease in small ruminants, are highly niche-restricted. However, our understanding of the genetic basis for such differences in bacterial host-tropism is very limited. Here, we discovered that S. aureus subsp. anaerobius evolved from an S. aureus ancestor and underwent an array of extensive changes to its genome that accompanied the transition to its current restricted lifestyle. We observed genome decay involving loss of function of hundreds of genes, large intra-chromosomal rearrangements affecting most of the genome, acquisition of a pathogenicity island, and expansion of large numbers of insertion sequences that are inserted at intergenic sites around the genome. Importantly, we found that IS elements affect the expression of neighbouring genes in different ways including a novel mechanism of IS-enabled disruption of ancestral gene repression. Taken together, we provide a remarkable example of radical genomic changes associated with evolutionary transition from a multi-host to highly restricted host ecology.

Diagnostic pathology in microbial diseases of sheep or goats

Post-mortem examination is a key step in the diagnostic process of infectious diseases in sheep and goats. Diagnostic pathology deals with identification and study of lesions, at the same time providing also significant clues regarding pathogenesis of the diseases. This article reviews the salient pathological findings associated with the most significant infectious diseases of sheep and goats present in countries where small ruminants are a relevant agricultural industry. Lesions are reviewed according to the different organ systems where they occur. Emphasis has been given in the description of the salient lesional patterns than can be identified in each organ and which can be of help in the differential diagnosis of the lesions caused by bacteria, viruses, fungi or prions. Finally, a review of the usefulness of ancillary tests that may be used on various tissue samples for performing an aetiological diagnosis, is included; the application of various techniques, from immunohistochemistry to molecular biology-based tests, is described.

Restoring catalase activity in Staphylococcus aureus subsp. anaerobius leads to loss of pathogenicity for lambs

Staphylococcus aureus subsp. anaerobius, a microaerophilic and catalase-negative bacterium, is the etiological agent of abscess disease, a specific chronic condition of sheep and goats, which is characterized by formation of necrotic lesions that are located typically in superficial lymph nodes. We constructed an isogenic mutant of S. aureus subsp. anaerobius (RDKA84) that carried a repaired and functional catalase gene from S. aureus ATCC 12600, to investigate whether the lack of catalase in S. aureus subsp. anaerobius plays a role in its physiological and pathogenic characteristics. The catalase activity had no apparent influence on the in vitro growth characteristics of RDKA84, which, like the wild-type, did not grow on aerobically incubated agar plates. Restoration of catalase activity in RDKA84 substantially increased resistance to H₂O₂ when analyzed in a death assay. The intracellular survival rates of the catalase-positive mutant RDKA84 in polymorphonuclear neutrophils (PMN) isolated from adult sheep were significantly higher than those of the wild-type, while no differences were found with PMN isolated from lambs. RDKA84 showed significantly lower survival rates in murine macrophages (J774A.1 cells) than the wild-type strains did, whereas, in bovine mammary epithelial cells (MAC-T), no differences in intracellular survival were observed. Interestingly, the virulence for lambs, the natural host for abscess disease, of the catalase-positive mutant RDKA84 was reduced dramatically in comparison with wild-type S. aureus subsp. anaerobius in two experimental models of infection.

Abscess disease, caseous lymphadenitis, and pulmonary adenomatosis in imported sheep

The occurrence of abscess disease, caseous lymphadenitis, and pulmonary adenomatosis in sheep in Denmark is reported for the first time. Subcutaneous abscesses were observed in imported 4- to 5-month-old lambs of the Lacaune breed 10 days after arrival in Denmark. Abscesses were mostly located in the head, neck and shoulder regions close to the regional lymph nodes. Bacteriological examinations revealed growth of Staphylococcus aureus ssp. anaerobius in all animals with subcutaneously located abscesses containing a viscous white-yellow odourless mass. In addition, Corynebacterium pseudotuberculosis was isolated from abscesses in one animal and lesions consistent with pulmonary adenomatosis were found in four animals.

Catalase deficiency in Staphylococcus aureus subsp. anaerobius is associated with natural loss-of-function mutations within the structural gene

Degenerate oligonucleotide primers based on internal peptide sequences obtained by HPLC from purified Staphylococcus aureus catalase were used to locate the S. aureus and S. aureus subsp. anaerobius kat regions by PCR. Southern hybridization analysis with a probe derived from a 1.1 kb PCR-amplified fragment showed that a single copy of the putative catalase gene was present in the S. aureus and S. aureus subsp. anaerobius chromosome. The nucleotide sequence of S. aureus katA revealed a 1518 bp open reading frame for a protein with 505 amino acids and a predicted molecular mass of 58347 Da, whereas S. aureus subsp. anaerobius katB is 1368 nt long and encodes a polypeptide of 455 amino acids with a predicted molecular mass of 52 584 Da. These catalases are highly homologous to typical monofunctional catalases from prokaryotes. The active-site residues, proximal and distal haem-binding ligands and NADPH-binding residues of the bovine liver catalase-type enzyme were highly conserved in S. aureus KatA. Escherichia coli cells carrying cloned katA had a catalase activity approximately 1000 times that of untransformed E. coli, but no detectable increase in catalase activity was observed with E. coli carrying cloned katB. Northern blotting showed the presence of a kat-specific transcript in S. aureus subsp. anaerobius, suggesting that the lack of catalase activity in this bacterium is due to a post-transcriptional alteration. Compared to the nucleotide sequence of katA, katB showed a single base-pair deletion and six mis-sense mutations, and these alterations were present in three other S. aureus subsp. anaerobius strains analysed. The deletion, located at 1338 bp from the initiation codon, originates a shift of the nucleotide reading frame and is responsible for the premature translation termination at 1368 bp, generating a KatB polypeptide 50 amino acid residues shorter than KatA. Moreover, four of the mis-sense mutations present in katB lead to non-conservative amino acid replacements, the most significant being that located at residue 317 (Pro in KatA-->Ser in KatB) because the affected amino acid is involved in determining the proximal haem-binding site. Both the main alterations found in KatB (the deletion and the substitution in residue 317) seem to contribute to the lack of catalase activity in S. aureus subsp. anaerobius, as deduced from results obtained with chimeric catalase constructs.

Identification of coagulase negative staphylococci isolated from lambs as Staphylococcus caseolyticus

A group of 17 strains of coagulase negative staphylococci isolated from slaughtered lambs, and which could not be identified with the conventional methods, exhibited high levels of DNA homology (92%) with the S. caseolyticus reference strain. The isolates described in this study provide a more extensive comprehension of S. caseolyticus. The original description of this species was based on only two strains isolated from milk. To our knowledge, S. caseolyticus had never been previously associated with animal microflora.