Article

Characteristics of Spore Germination in a Mouse Model of Cutaneous Anthrax

Division of Infectious Diseases, Department of Medicine, Medical College of Wisconsin, Milwaukee, WI, 53295, USA.
The Journal of Infectious Diseases (Impact Factor: 6). 04/2007; 195(6):888-94. DOI: 10.1086/511824
Source: PubMed

ABSTRACT

Cutaneous infection is the most common form of human anthrax, but little is known about Bacillus anthracis spore germination in these infections.
We used experimental inoculations of B. anthracis Sterne spores or vegetative bacilli onto intact or abraded mouse flank skin, followed by evaluation of the infections and enumeration of germinating spores and vegetative bacilli.
Bacilli developed from a spore inoculum after application onto abraded, but not intact, skin of the mice. Germination appeared to occur extracellularly at the skin surface before the development of a phagocytic response; in fact, vegetative bacilli were seen after inoculation of the spores on top of a filter that separated them from the host phagocytic cells below. Malachite green staining demonstrated that spores began germinating 1-3 h after inoculation onto abraded skin. Vegetative bacilli were found not to be capable of initiating infection in the absence of cutaneous abrasion.
The results indicate that epidermal damage is required for germination of B. anthracis spores in these infections; even so, spore germination by itself is not sufficient to produce infection of undamaged skin. In contrast to events in experimental inhalational anthrax, spore germination in these cutaneous infections appears to occur extracellularly.

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    • "Upon entering a potential host, spores germinate into vegetative bacilli that replicate and disseminate through the bloodstream, leading to a systemic infection. Whereas in experimental inhalational anthrax, spore germination requires engulfment by macrophages/DCs (Guidi-Rontani et al., 1999; Hanna and Ireland, 1999), in experimental cutaneous infections it has been shown that most of the spore germination occurs extracellularly (Bischof et al., 2007; Corre et al., 2013). Soon after spore germination, vegetative B. anthracis start producing two potent exotoxins – anthrax lethal toxin (LT) and edema toxin (ET) – that along with a poly-D-glutamic acid capsule, are its major virulence factors (Moayeri et al., 2015). "

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    • "On the other hand, these organisms do not require an initial period of germination as does B. anthracis and therefore could possibly invade damaged skin before an initial neutrophilic response would clear them. Germination of B. anthracis spores is quite rapid in appropriate media and we have found that this process is well underway within 1–3 h after inoculation in this model system (Bischof et al. 2007b). However, it may be that even a short interval like this is enough to shift the kinetics of B. anthracis infection towards clearance by the host inflammatory cells, particularly for non-encapsulated strains. "
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    ABSTRACT: Skin window procedures in humans have shown rapid accumulation of neutrophils into the exuded fluids above abraded skin. The present study was undertaken to determine if similar epicutaneous neutrophil accumulation might explain the extreme resistance of HRS/J mice, both hairless (hr/hr) and haired (hr/+), to experimental cutaneous Bacillus anthracis Sterne infections on abraded skin. In this study, very early (6 h) biopsies demonstrated a lack of bacilli in skin from the HRS/J hr/hr mice, indicating that the organisms never did invade in these animals as opposed to early skin entry and then efficient clearance by host responses in the tissues. Touch preparations of either the inoculation filter or the skin surface revealed more inflammatory cells, fewer bacilli, and a higher percentage of cell-associated bacilli in the HRS/J hr/hr mice than in comparator strains. In the HRS/J mice, cyclophosphamide treatment or separation of inoculated spores from the inflammatory infiltrates by a second filter below both produced marked increases in the number of bacilli observed. Examination of inoculation filter specimens demonstrated ingestion of spores and bacilli by neutrophils inside the filter at 6 h after inoculation. These findings suggest that an early and vigorous inflammatory cell infiltrate in HRS/J mice attacks the inoculated organisms above the skin surface and does not allow them to invade the tissues below.
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