Gram positive bacteria
Gram positive bacteria are a diverse group. The most clinically important gram ositive pathogens include Staphylococcus, Streptococcus and Entercocci.
Gram-positive cocci
Enterococcus: are gram positive cocci to elongated cocci in single or short chains. The are catalase negative, facultatively anaerobic. The Enterococci family can be found as part of the normal flora of the gastrointesitnal 30 tracts of humans and other animals. Over the last 30 years, Enterococci have become major nosocomial pathogens. They can cause infections at a variety of sites, including the urinary tract, bloodstream, endocardium, abdomen and biliary tract, as well as burn wounds and indwelling devices. Among the 15 species of Enterococci, E faecalis and E faecium are the most commonly associated with clinical infection.
–Enterococcus faecalis: causes mostly nosocomial or opportunistic urinary tract infections, wound infections and bacteremia in seriously ill elderly pateints.
–Enterococcus faecium: E. faecium has rapidly evolved as a worldwide nosocomial pathogen by successfully adapting to conditions in a nosocomial setting and acquiring resistance against glycopeptides. They have acquired a number of traits making them successful in the hospital environment; such as an increase in antibiotic resistance genes and virulence genes enhancing biofilm formation and colonization
Micrococcus: are gram positive cocci in pairs, tetrads or clusters which are catalase positive, strictly aerobic and does not produce acid from glucose.
–Micrococcus luteus: is an opportunistic infection of immunocompromised patients.
Staphylococci (Staphylococcus): are (so gram test can be used to identify) cocci which are most commonly found in clusters (another identification clue that distinguishes them from say Strept).
Because stophylococci are carried on the skin surface and in the nasopharynx, shedding of the bacteria is common and is responsible for many hospital acquired infections. They are sususpectable to disinfectants and antiseptic solutions but are capable of survival on dry surfaces for long periods. Transfer to a susceptible individual can be either by direct contact or by means of fomits (contaminated clothing or bed lines).
Treatment for staphylococcal infections include (like cephalosporin) although plasmids encoding beta-lactamases are common.
The only species of Staphylococci which produces the enzyme coagulase is S. aureus whereas all other species are coagulase negative (a distinguishing characteristic).
Group A streptococci are responsible for a wide variety of human diseases, the most common of which are nasopharyngitis and imeptigo. Nearly all clinical isolates have the antiphagocytic factor, M protein, on their surface. This virulence factor displays extreme antigenic diversity within its amino terminal region. It is these highly variable poritions of M proteins which form the basis of the serological type whchme which was formulated in the 1930s perior to knowledge of any structural detail. Group A streptococci can be divided into two major classes partly on the basis of their immunoreactivity with a pair of monoclonal antibodies directed to eptiopes which lie within the relatively conserved half of M proteins. Class I isolates are defined as those binding one or both monoclonal antiboides, whereas class II isolates do not bind either monoclonal antibody. In addition, the classes differ in their ability to exhibit opacity factor activity, and in several pathogenic properties of these organisms (US 5,556,944).
–S. epidermidis are circular, white, arranged in grape-like clusters, raised on sheep blood agar, coagulase negative (distinguish S. aureus above), non-hemolytic, catalase positive, facultative anaerobes. They are part of the human skin flora and can be found in mucous membranes. They are opportunisitc in that people with compromised immune systems are at risk for developing an infection.
Streptoccoci are and may occur in pairs or as chains. They are catalase negative (unlike Staphylococcus) and most species are . 4 different schemes are used for classifying these organisms: 1) clinical presentation (pyogenic, oral, enteric);2) serological properties (Lancefield groupings); 3) hemolytic patterns (complete B hemolysis, incomplete alpha hemolysis, and no gamma-hemolysis) and 4. biochemical properties.
–Streptococcus intermedius: is a Gram-positive bacteria that can cause brain and liver abscesses. It secretes a member of the cholesterol-dependent cytolysis (CDC) toxin family called Streptococcus intermedius intermedilysin (ILY). CDC play various roles in the pathogenesis of a wide variety of human and animal diseases caused by Gram positive bacterial. Human CD59 is a receptor for the cholesterol-dependent cytolysin intermedilysin (Giddings, Nature Structural & molecular biology, 2004).
–Streptococcus pyogenes (S. pyogenes)
–Streptococcus agalactiae (Group B Streptococcus): cause newborn meningitis and pneumonia. Colonies are larger than Group A, and beta hemolysis zone is smaller. Group B Streptococci requires . They have an outermost capsule like Group A. They colonize the upper respiratory tract, lower GI tract, and vagina. Streptococcus agalactiae (Group B Streptococcus, GBS) is an important human pathogen that colonizes the urogenital and/or the lower gastro-intestinal tract of up to 40% of healthy women of reproductive age and is a leading cause of sepsis and meningitis in the neonates. GBS can also infect the elderly and immuno-compromised adults, and is responsible for mastitis in bovines. Like other Gram-positive bacteria, GBS can form biofilm-like three-dimensional structures that could enhance its ability to colonize and persist in the host. Biofilm formation by GBS has been investigated in vitro and appears tightly controlled by environmental conditions.See Mayo Clinic
–Viridan Streptococci are alpha and non-hemolytic streptococci. They are most often associated with dental caries.
Gram + Bacilli
Bacillus: Bacillus species are which may form chains. They are spore formers and some have capsules. They require aerobic growth and can be grown on blood agar. The pathogenic ones secrete exotoxins.
Bacillus is a very diverse genus with more than 200 species, and the identification and diagnosis of potential disease-causing Bacillus species from patient material by the clinical lab can be challenging. This is especially the case for members of the B. cereus family. Despite their different species names, all these bacteria are often considered to be members of one species based on their high content of shared genes. See Cote
–B. anthracis (anthrax):
–B. cereus: are rod shaped, beta hemolytic, facultative anaerobes and can produce endospores. Bacillus cereus is an endospore-forming, gram-positive bacterium that commonly resides in the soil. Because of its location, Bacillus cereus is usually found on a variety of foods that come into close contact with contaminated soil, and can cause two different types of food borne illnesses: emetic and diarrheal. They are endemic and soil dwelling. Consequently, food products of plant origin frequently contain their spores. There is a marked seasonal variation in the spore content of raw milk, with higher levels during the pasture period, when the cows tetes may contain their spores from soil. Dirty teats that are not cleansed before milking are an important contamination source, particularly during wet weather. Some strains cause food poisoning which occurs due to survival of the bacterial endospores when food is improperly cooked. They can also cause opportunistic infections. After the ingestion of such contaminated foods, Bacillus cereus may incubate inside the infected organism for up to six hours before showing any symptoms that the emetic form displays like nausea, vomiting and abdominal cramps. Strains that produce the diarrheal form of the disease grow in a variety of foods ranging from vegetables to even meat products. Patients experience profuse diarrhea with abdominal pain and cramps…which begin around eight to sixteen hours after ingestion of the contaminated food. B. cereus illness is related to many foods – beef, turkey, rice, beans, vegetables. Specifically, the diarrheal illness is often related to meats, milk, vegetables, and fish. The emetic-type illness is most often associated with rice products, but it has also been associated with other types of starchy products such as potato, pasta, and cheese products. See Medic8.
Many strains of B. cereus are able to produce toxins and cause distinct types of food poisoning. Concerns over B. cereus contamination have increased because of the rapidly expanding number of chilled foods that may be pasteurized but may still contain viable spores. Spores from B. cereus can germinate and outgrow during storage, even at low temperatures. Major efforts to battle this increasing problem are focusing on determining the causes of the spore’s resistance and the mechanisms of germination. See Vries
While B. cereus is associated mainly with food poisoning, it is being increasingly reported to be a cause of serious and potentially fatal non-gastrointestinal-tract infections. The pathogenicity of B. cereus, whether intestinal or nonintestinal, is intimately associated with the production of tissue-destructive exoenzymes. Among these secreted toxins are four hemolysins, three distinct phospholipases, an emesis-inducing toxin, and proteases. The major hurdle in evaluating B. cereus when isolated from a clinical specimen is overcoming its stigma as an insignificant contaminant. Outside its notoriety in association with food poisoning and severe eye infections, this bacterium has been incriminated in a multitude of other clinical conditions such as anthrax-like progressive pneumonia, fulminant sepsis, and devastating central nervous system infections, particularly in immunosuppressed individuals, intravenous drug abusers, and neonates. See Botone
–B. clausii is a commonly used Bacillus spp. probiotic. Clinical data support its use for the treatment and prevention of gut barrier impairment. Small trials have investigated use in preterm neonates to prevent infection, treatment of nasal allergies and upper respiratory infections in children, and treatment of acute or chronic diarrhea, small-intestine bacterial overgrowth (SIBO), and adverse effects of Helicobacter pylori therapy in adults. See Drugs.com. Probiotic milk has been previously demonstrated to reduce the number of respiratory infections (RI) among children attending day care centres. See Marseglia
However caution should be exercised because Commensal/normal flora which are otherwise termed as ‘good bacteria’ are now causing infections in different group of patients, mostly immunocompromised individuals. Various host and environmental factors play a pivotal role in microbial transmigration from their normal habitat into the blood and other body sites. This has been reported for B. clausii. See Princess
–B. subtillis: is rod shaped, catalase positive, and can form a protective endospore. It is found in the soil and also considered as a normal human gut commensal. B.subtilis is non-pathogenic but can contaminate food and be considered an opportunistic pathogen among the immuno-compromised. They are used on seeds, vegetables, and plants as a fungicide because of their ability to produce antibiotics. B.subtilis inhabits the root system of the plant completing with disease causing organisms. Some B. subtilis strains are capable of producing toxins for insects. These strains are used by farms to protect their crops. See Microchem Laboratory
Clostridium (see outline)
Lactobacillus: are gram positive rods, sometimes in chains. They are catalase engative and have no endospores. They are isolated form a variety of foods (dairy, fish, grain, meat) and many are in normal flora (mouth, intestines, and vagina). There are no key pathogens in the clinial setting
Lysteria
–Listeria monocytogenes: is a ubiquitous, facultative intracellular, gram positive bacillus that causes listeriosis, an often fatal disease in newborns, pregnant women, and the immunocompromised. It is one of the most virulent food born pathogens. See CDC website
Mycobacteria: are obligate, . They are called “acid fast” in that they are very resistent to destaining after you stain them due to their complex cell wall lipids (mycolic acids). They are slow growing organisms and produce pigment depending on the presence or absence of light. Once inside a cell, mycobacteria are resistent to phagoccytosis. Some important types of mycobacteria are the following:
–M. leprae: occurs mostly in the tropics. Diagnosis is by way of a skin test reagent (lepromin), extracted from autoclaved tissue harvested from patients with leprosy. Therapy is with dapsone which has to be administered for years.
–M. avium is rare in immunocompetent people but a common opportunistic bacterial infection in patients. M. avium complex (MAC) is the most common cause for both disseminated Mycobacterium disease and death in patients with AIDS in the developed world (~25%-50% of adults and 10% of children with AIDS are infected).
–M. kansasii is a pulmonary infection, resembling TB. It has a photoinducible pigment.
–M. marinum are acquired by contact with contaminated fresh water or salt water. For this reason, they are called “swimming pool granulomas.” It responds well to antibiotics.
–M. fortuitum are most commonly associated with disease following the introduction of the bacteria into deep subcutaneous tissues by trauma or intravenous catheter, contaminated wound dressing, etc. Infections with these organisms are increasing as more invasive procedures are performed on hospitalized patients. These organisms are faster growing than other mycobacteria.
Gram Positive Bacteria that Infects Non-Humans
Paenibacillus larvae is a Gram-positive, spore-forming bacterium that is the causative agent of American foul brood (AFB), the most devastating bacterial disease of the honeybee. P. larvae is antibiotic resistant, complicating treatment efforts. Bacteriophages that target P. larvae are rapidly emerging as a promising treatment. See Genon