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GRAM POSITIVE RODS The Gram-positive rods in this section will be divided into three distinct varieties based upon their ability to produce endospores and their morphological appearance: ENDOSPORE-FORMING BACILLUS REGULAR, NON-ENDOSPORE-FORMING LACTOBACILLUS LISTERIA ERYSIPELOTHRIX IRREGULAR, NON-ENDOSPORE-FORMING CORYNEBACTERIUM These bacteria are ubiquitous in nature and most are aerobic or facultatively anaerobic. BACILLUS Bacillus represents a genus of Gram-positive bacteria which are ubiquitous in nature (soil, water, and airborne dust). Some species are natural flora in the human intestines. When grown on blood agar, Bacillus produces large, spreading, gray-white colonies with irregular margins. A unique characteristic of this bacterium is its ability to produce endospores when environmental conditions are stressful. The only other known spore-producing bacterium is Clostridium. Although most species of Bacillus are harmless saprophytes, two species are considered medically significant: B.anthracis and B. cereus. B. anthracis B. anthracis is the bacterium which causes anthrax in cows, sheep, and sometimes humans. Anthrax is transmitted to humans via direct contact with animal products or inhalation of endospores. Under the microscope, B. anthracis cells appear to have square ends and seem to be attached by a joint to other cells. The spores are best observed when the bacterium is cultered on artificial media. Sources of infection are usually industrial or agricultural and the infection is classified as one of three types: CUTANEOUS INFECTION (95% of human cases) INHALATION ANTHRAX (rare) GASTROINTESTINAL ANTHRAX (very rare!) LABORATORY INDICATIONS: Nonhemolytic (sheep blood agar) Non-motile Gel hydrolysis - Catalase + B. cereus Unlike B. anthracis, B.cereus is a motile bacterium which can cause toxin-mediated food poisoning. It is known to inhabit many kinds of food including stew, cereal, and milk. Most recently, however, it has been found in fried rice. The two toxins released by the bacterium lead to vomiting and diarrhea, symptoms similar to those of Staphylococcus food poisoning. Because toxin production usually takes place after the infected foods are cooked, proper cold storage of food is recommended immediately after preparation. LABORATORY INDICATIONS: Hemolytic (sheep blood agar) Motile Gel hydrolysis + Glucose, maltose, & salicin fermentative Catalase + LACTOBACILLUS Most species of this non-spore-forming bacterium ferment glucose into lactose, hence the name Lactobacillus. The most common application of Lactobacillus is industrial, specifically for dairy production. This genus also contains several bacteria that make up part of the natural flora of the human vagina. Because of their ability to derive lactic acid from glucose, these bacteria create an acidic environment which inhibits growth of many bacterial species which can lead to urogenital infections. Lactobacillus is generally harmless to humans, rarely inciting harmful infections or diseases. Treatment of this vancomycin-resistant microbe usually consists of high doses of penicillin in combination with gentamicin. LABORATORY INDICATIONS: Catalase - Lactic acid production from glucose Growth on tomato juice agar LISTERIA Listeria is a Gram-positive rod which is not capable of forming endospores. Although several species of this bacterium exist, our discussion will focus only on the two species of human pathogenic significance: L. monocytogenes and L. ivanovii. In particular, L. monocytogenes has been implicated in several food poisoning epidemics. This normal inhabitant of the gastrointestinal tract and of animal feces led to a 1986 outbreak in Massachusetts hospital patients. Those infected suffered from vomiting, nausea, and diarrhea. Apparently, the hospital patients contracted the microbe from the infected hospital food and were at high risk of infection. Those at high risk include newborns, pregnant women and their fetuses, the elderly, and persons lacking a healthy immune system. The bacterium usually causes septicema and meningitis in patients with supressed immune function. It also causes listeriosis which is an inflammation of the brain. Antibiotics are recommended for treatment of infection because most strains of Listeria are sensitive to ampicillin and gentamicin. LABORATORY INDICATIONS: Catalase + Motile at room temperature Growth at 4 degrees Celsius Bile esculin hydrolysis Beta-hemolysis ERYSIPELOTHRIX E. rhusiopathiae, the only species of this genus, is better known as a veterinary pathogen than as a human pathogen. When cultured on blood agar or some other nutrient medium, Erysipelothrix forms notably large colonies. This ubiquitous microbe has been found in many farm animals such as pigs, horses, and turkeys. Occasionaly, though, it can infect a human host and cause an inflammatory skin disease, Erysipeloid. Treatment usually consists of penicillin G, ampicillin, or cephalothin. Most clinical strains have been found to be resistant to the super-antibiotic, vancomycin. LABORATORY INDICATIONS: Catalase - Non-motile TSI : H2S + CORYNEBACTERIA The coryneform group of Gram-positive bacteria includes several genera of non-spore-forming rods which are ubiquitous in nature. We will consider only two of these genera in our discussion of clinically significant microorganisms: Actinomyces and Corynebacterium. The first genus, Actinomyces, will be presented later when we focus on anaerobic Gram-positive bacteria. The second genus, Corynebacterium, is comprised of facultatively anaerobic bacteria which are normally saprophitic and harmless to humans. An exception is the bacterium C. diphtheriae which produces the toxin that causes diphtheria, a disease of the upper respiratory system in humans. Under the microscope (best viewed using Loeffler's methylene blue dye), C. diphtheria can be seen forming colonies which clump up or stick together. This is a characteristic associated with many higher forms of bacteria. Although other species of Corynebacterium can inhabit the mucous membrane, C.diphtheria is unique in its exotoxin formation. Treatment for the disease usually consists of administration of an antitoxin with penicillin. LABORATORY INDICATIONS: Catalase + Nitrate + Glucose fermentation Non-motile