Pathogenic Bacteria and Fungi and Thorns

Three recent publications showed that spines harbor an array of pathogenic bacteria and fungi (Halpern et al. 2007a, b; Lev-Yadun and Halpern 2008). Spines from date palm (Phoenix dactylifera) trees, thorns from common hawthorn (Crataegus aronia) trees and two thorny shrub species, thorny burnet (Sarcopoterium spinosum) and manna tree (Alhagi graecorum), were sampled in Israel. Every typical mature individual of these trees and shrubs carries hundreds or even thousands of conspicuous and therefore potentially aposematic spines or thorns. The severity and frequency of infections among orchard workers in Israel following date-palm spine wounding has necessitated the costly practice of removing all of the millions of spines from many of the orchards using mechanical saws. Even the small number of spines and thorns studied resulted in a list of aerobic and anaerobic bacteria species including Clostridium perfringens, Bacillus anthracis and Pantoea agglomerans (Halpern et al. 2007a, b).

C. perfringens is known to be a flesh-eater in that it can produce a necrotizing infection of the skeletal muscle called gas gangrene (Shimizu et al. 2002) . Clostridium tetani, the etiological agent of tetanus, a serious disease in humans and animals, can be fatal when left untreated. Thorn injuries have been known to cause tetanus in the USA, Ethiopia, and Turkey (Hodes and Teferedegne 1990; Ergonul et al. 2003 ; Pascual et al. 2003) . B. anthracis is the etiological agent of anthrax, a notoriously acute fatal disease in both domesticated and wild animals, particularly herbivorous ones, and humans (Jensen et al. 2003). The cutaneous form of the disease is usually acquired through injured skin or mucous membranes, a typical thorn injury. None of the published medical data discussed ecological or evolutionary issues or aposema-tism, but were instead only published in the interests of medical practice. However, these data showed that plant thorns, spines and prickles may regularly harbor various toxic or pathogenic bacteria (Halpern et al. 2007a, b).

In their review of the medical literature, Halpern et al. (2007b) found that septic inflammation caused by plant thorn injury can result from not only bacteria but also pathogenic fungi. Dermatophytes that cause subcutaneous mycoses are unable to penetrate the skin and must be introduced into the subcutaneous tissue by a puncture wound (Willey et al. 2008).

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