Abstract
ObjectiveCharacterization of Staphylococcus aureus clinical isolates derived from lower respiratory tract infections (LRTIs), and correlation between the functionality of the accessory gene regulator (Agr) and genotypic and phenotypic characteristics, clinical variables and clinical outcome.MethodsS aureus isolates derived from LRTIs and control groups (nasal carriage and bacteraemia) were genotyped using StaphyType DNA microarray. Agr activity was evaluated using the CAMP synergistic haemolysis assay and the Vesicle Lysis Test (VLT). Discordant strains were analysed by quantitative reverse-transcriptase real-time PCR (qRT-PCR).ResultsAgr was functional in 79.7% and 84.5% of strains according to the CAMP and VLT assays respectively. Higher concordance with RNAIII expression measured by qRT-PCR was observed with the VLT assay (76.2%) compared with the CAMP assay (23.8%). No statistically significant differences were observed in Agr functionality between the study groups, nor the phenotypical/genotypical bacterial characteristics. No association between increased mortality/respiratory complications and Agr function was observed.ConclusionsAgr activity was high (82.2%) in isolates from LRTIs suggesting the importance of this global regulator in lower respiratory tract colonisation and infection. However, equally high Agr activity was observed in isolates derived from nasal carriage and bacteraemia, contradictory to previous observations. Agr functionality measured by the VLT assay was superior to CAMP assay.
Highlights
Staphylococcus aureus is a commensal and an opportunistic pathogen, described as the causative agent in a wide range of human infections [1]
accessory gene regulatory (Agr) was functional in 79.7% and 84.5% of strains according to the CAMP and Vesicle Lysis Test (VLT) assays respectively
Higher concordance with RNAIII expression measured by quantitative reverse-transcriptase real-time PCR (qRT-PCR) was observed with the VLT assay (76.2%) compared with the CAMP assay (23.8%)
Summary
Staphylococcus aureus is a commensal and an opportunistic pathogen, described as the causative agent in a wide range of human infections [1]. The accessory gene regulatory (Agr) system is one of the most important and well-characterised operons in S. aureus biology, central in the control and regulation of virulence gene expression [6,7,8,9]. Activation of the P2 promoter drives the expression of the components of the quorum-sensing system (AgrBDCA). Airway colonization by S. aureus is a precursor for the development of LRTI, little is known about the pathogen-associated factors that promote progression from colonization to LRTI [12]. It has been widely documented that a functional Agr is central for causing disease in animal models of infection [13,14,15]. Agr dysfunctional strains generally have a higher biofilm capacity and are more fit in vitro due to the large metabolic burden of having an active Agr system [22, 23]
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