From clinical microbiology to infection pathogenesis: how daring to be different works for Staphylococcus lugdunensis

Abstract

Staphylococcus lugdunensis has gained recognition as an atypically virulent pathogen with a unique microbiological and clinical profile. S. lugdunensis is coagulase negative due to the lack of production of secreted coagulase, but a membrane-bound form of the enzyme present in some isolates can result in misidentification of the organism as Staphylococcus aureus in the clinical microbiology laboratory. S. lugdunensis is a skin commensal and an infrequent pathogen compared to S. aureus and S. epidermidis, but clinically, infections caused by this organism resemble those caused by S. aureus rather than those caused by other coagulase-negative staphylococci. S. lugdunensis can cause acute and highly destructive cases of native valve endocarditis that often require surgical treatment in addition to antimicrobial therapy. Other types of S. lugdunensis infections include abscess and wound infection, urinary tract infection, and infection of intravascular catheters and other implanted medical devices. S. lugdunensis is generally susceptible to antimicrobial agents and shares CLSI antimicrobial susceptibility breakpoints with S. aureus. Virulence factors contributing to this organism's heightened pathogenicity remain largely unknown. Those characterized to date suggest that the organism has the ability to bind to and interact with host cells and to form biofilms on host tissues or prosthetic surfaces.

FIG. 1.

Key biochemical characteristics of S. lugdunensis. (A) Tube coagulase (long coagulase) test. S. aureus RN4220 (bottom) clots plasma (BBL rabbit coagulase plasma with EDTA; BD, Franklin Lakes, NJ), whereas S. lugdunensis IDRL-5258 (top) is unable to clot plasma. (B) Ornithine decarboxylase test. S. aureus RN4220 (left) and S. lugdunensis IDRL-5258 (right) were used to inoculate decarboxylase medium agar (BBL Moeller decarboxylase broth base; BD) containing ornithine and overlaid with mineral oil. S. lugdunensis demonstrates ornithine decarboxylase activity, causing a change in the medium's pH indicator (bromocresol purple) to a purple color, signifying a positive result. In contrast, a negative result (yellow) is obtained with S. aureus. (C) Rapid PYR test. S. lugdunensis IDRL-5258 (bottom) produces PYR, exhibiting a positive (pink) reaction in a rapid test (Remel, Lenexa, KS). S. aureus RN4220 (top) is PYR negative. (D) Synergistic hemolysis. The δ-like hemolysin of S. lugdunensis, mediated by three small peptides produced upon expression of the slush locus (see text for details), acts synergistically with the β-hemolysin of S. aureus to produce a zone of complete hemolysis on sheep red blood cells. S. lugdunensis IDRL-5258 (left) and S. lugdunensis IDRL-2414 (right, streaked in triplicate) are streaked perpendicular to, but not touching, S. aureus RN4220, which is streaked vertically. In panels A, B, and D, photographs were taken 18 h after inoculation and incubation in ambient air at 37°C. Panel C was photographed approximately 5 minutes after the rapid test was performed.

FIG. 2.

Microscopic visualization of S. lugdunensis biofilms. (A) Scanning electron micrograph (magnification, ×11,000) of S. lugdunensis IDRL-2640 biofilm formed on a silicone elastomer disk (Bentec Medical, Woodland, CA) after 24 h. Nonadherent cells were rinsed from the disk following incubation, and the disk was placed in Trumps fixative (4% formaldehyde and 1% glutaraldehyde in phosphate buffer, pH 7.3). Images were acquired on a Hitachi S-4700 instrument (Hitachi Ltd., Tokyo, Japan) by cold-field emission microscopy following critical-point drying and gold-palladium sputter coating. Bar, 5 μm. (B) Confocal scanning laser microscopy image of an S. lugdunensis IDRL-2640 biofilm grown in a chambered coverglass well (Lab-Tek II; Nalge Nunc International, Rochester, NY) for 24 h. Cells (green) were stained with the nucleic acid stain Syto-9 (Molecular Probes, Eugene, OR), and extracellular proteins (red) were visualized with Sypro Ruby (Molecular Probes). Fluorophores were excited with an argon laser at 458 nm (Sypro Ruby) and 488 nm (Syto-9). Fluorescence was detected from Sypro Ruby with an LP 650 filter and from Syto-9 with a BP 505-530 filter. Images were collected on an LSM410 unit equipped with an Axiovert 100 M inverted microscope using a Plan-Apochromat 100×/1.4 NA oil immersion objective (Carl Zeiss, Inc., Thornwood, NY). Bar, 5 μm.