Medical Microbiology Lec.7 PDF

Lec.7 Assist Prof. Dr. Mohammed Alrufae Medical Microbioilgy Bacillus species  The genus Bacillus includes large aerobic, gram-positive rods occurring in chains.  The members of this genus are closely related but differ both phenotypical...

Lec.7 Assist Prof. Dr. Mohammed Alrufae Medical Microbioilgy Bacillus species  The genus Bacillus includes large aerobic, gram-positive rods occurring in chains.  The members of this genus are closely related but differ both phenotypically and of pathogenesis.  Pathogenic species possess virulence plasmids.  Most members of this genus are saprophytic organisms prevalent in soil, water, and air, and on vegetation (eg, Bacillus subtilis).  Some are insect pathogens, such as ( B. Thuringiensis) this species is also causing disease in humans.  B. cereus can grow in foods and cause food poisoning by producing either an enterotoxin (diarrhea) or an emetic toxin (vomiting).  Both B. cereus and B. thuringiensis may occasionally produce disease in immunocompromised humans (eg, meningitis, endocarditis, acute gastroenteritis).  B anthracis, which causes anthrax, is the principal pathogen of the genus.  spores are located in the center of the bacilli.  The spores are resistant to environmental changes, withstand dry heat and certain chemical disinfectants for moderate periods, and persist for years in dry earth.  Animal products contaminated with anthrax spores can be sterilized by autoclaving. Bacillus anthracis  Anthrax is primarily a disease of herbivores goats, sheep, cattle, horses other animals (eg, rats) are relatively resistant to the infection.  Humans become infected incidentally by contact with infected animals or their products.  In animals, the portal of entry is the mouth and the gastrointestinal tract.  Spores from contaminated soil find easy access when ingested with spiny or irritating vegetation.  Lec.7 Assist Prof. Dr. Mohammed Alrufae Medical Microbioilgy  In humans, the infection is usually acquired by the entry of spores through injured skin (cutaneous anthrax) or rarely the mucous membranes (gastrointestinal anthrax) or by inhalation of spores into the lung (inhalation anthrax).  The spores germinate in the tissue at the site of entry, and growth of the vegetative organisms results in formation of a gelatinous edema and congestion.  Bacilli spread via lymphatics to the bloodstream, and they multiply freely in the blood and tissues shortly before and after the animal’s death.  B anthracis isolates that do not produce a capsule are not virulent and do not induce anthrax in test animals.  The poly-α-d-glutamic acid capsule is antiphagocytic.  Anthrax toxins are made up of three proteins, 1-protective antigen (PA), 2-edema factor (EF), 3-lethal factor (LF).  PA is a protein that binds to specific cell receptors, and after proteolytic activation, it forms a membrane channel that mediates entry of EF and LF into the cell.  EF is an adenylate cyclase; with PA, it forms a toxin known as edema toxin. responsible for cell and tissue edema.  LF plus PA form lethal toxin, which is a major virulence factor and cause of death in infected animals and humans. Resistance and Immunity  Active immunity to anthrax can be induced in susceptible animals by vaccination with 1- live attenuated bacilli, 2- with spore suspensions, 3- with PA from culture filtrates.  Lec.7 Assist Prof. Dr. Mohammed Alrufae Medical Microbioilgy Because the anthrax vaccines provide short-lived immunity and require repeated vaccinations, a number of new recombinant PA vaccines (rPA) have been developed. These vaccines have been shown to be very well tolerated and highly immunogenic. Treatment  Many antibiotics are effective against anthrax in humans, but treatment must be started early.  Ciprofloxacin is recommended for treatment; other agents with activity include penicillin G, doxycycline, erythromycin, and vancomycin.  the exposure to B anthracis as an agent of biologic warfare, prophylaxis with ciprofloxacin or doxycycline should be given for 60 days and three doses of vaccine should be administered. Bacillus cereus  Food poisoning caused by B cereus has two forms 1- the emetic type, which is associated with rice, milk, and pasta 2- the diarrheal type, which is associated with meat dishes and sauces.  B cereus produces toxins that cause disease that is more of intoxication than a food-borne infection.  (The emetic form) is manifested by nausea, vomiting, abdominal cramps, and occasionally diarrhea and is self-limiting, with recovery occurring within 24 hours.  It begins 1–5 hours after ingestion of a plasmid-encoded preformed cyclic peptide (emetic toxin) in the contaminated food products.  B cereus is a soil organism that commonly contaminates rice.  When rice are cooked and allowed to cool slowly, B cereus spores germinate, and the vegetative cells produce the toxin during log-phase growth or during sporulation.  Lec.7 Assist Prof. Dr. Mohammed Alrufae Medical Microbioilgy  (The diarrheal form ) has an incubation period of 1–24 hours and is manifested by profuse diarrhea with abdominal pain and cramps; fever and vomiting are uncommon.  B cereus is resistant to a variety of antimicrobial agents, including penicillins and cephalosporins. Serious non–food borne infections should be treated with vancomycin or clindamycin with or without an aminoglycoside. Ciprofloxacin has been useful for the treatment of wound infections.