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Microbiology 12 |
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Microbiology 12 |
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NOTE: INFORMATION ON THIS PAGE IS INTENDED FOR EDUCATIONAL USE ONLY. FOR MEDICAL ADVICE YOU SHOULD CONSULT WITH A PHYSICIAN.
The gram positive cocci are aerobic, facultative to strictly anaerobic in nature and are non-spore-forming. Pathogenic species are typically pyogenic (pus-forming). We will consider two medically important genera here: Staphylococcus and Streptococcus.
Staphylococcus aureus
Cultural and morphological characteristics
Staphylococcus refers to round cells arranged in clusters while the species name, aureus, refers to the golden appearance of colonies. S. aureus is a faculatative anaerobe and is distinguished from other Staphylococci by its ability to ferment mannitol, B-hemolysis of blood agar and production of coagulase (an enzyme that produces fibrin clots). Staphylococci are further distinguished from the genus Streptococcus as the former test catalase positive.
Habitat
Humans are the major reservoir of S. aureus; as many as 50% of adults and nearly all children carry the bacteria in their anterior nares. Staphylococci are also common skin flora and some strains colonize infants within 24 hours of birth. S. aureus is a leading cause of nosocomial infections and is easily transmitted by skin contact, nasal droplets and fomites (the organism survives on catheters, bedding, clothing and door knobs).
Diseases caused by strains of S. aureus
Perhaps no other organism has the ability to cause such a wide variety of infections ranging from food-poisoning to skin infections, toxic shock syndrome, septicemia and endocarditis. Many of the disease manifestations are produced by a wide variety of exoenzymes and exotoxins.
Skin infections Follicultis, boils and carbuncles involve invasion of
hair follicles or skin Impetigo is easily transmitted by skin contact and
handling objects such as toys and furniture Scalded Skin Syndrome is caused by certain
exotoxin-producing strains and is most common in infants Follicultis involves production of the familiar
pimple. A larger, deeper pustule is termed an
abscess. A raised abscess above the skin is a
furuncle (boil). An area of greater pyogenic
involvement is called a carbuncle. Impetigo is a very contagious pyogenic infection
that usually involves both staphylococci and streptococci.
Most infections occur in young children and involve
crusty-oozing lesions about the nose and mouth SSS-producing strains carry genes for one or two toxins
called exfoliatins. Infection usually starts around
the mouth. Toxins carried in the blood cause the upper skin
layers to separate and peel off. High fever and septicemia
are common. Death can occur in 36 hours of onset. Toxic shock syndrome Associatedwith the use of super-absorbent tampons in
menstruating women. Staphylococci increase in numbers in the
vagina of menstruating women. Several non-menstruating subsets of patients recognized
including persons undergoing nasal or wound surgery TSS is produced by strains of S. aureus that
secrete Toxic Shock Syndrome Toxin 1 (TSST-1). This
is an acute ilness that involves high fever, a diffuse red
skin rash and exfoliation of the skin of the hands and feet
(usually occurs 1-2 weeks after onset if not already fatal),
hypotension, shock and involvement of three or more organ
systems. Staphylococcal food poisoning This illness is an intoxication due to ingestion
of Staphylococcal enterotoxin. Warmed-over foods, ham, dairy products, custards and
potato salads and other protein foods are often
implicated Symptoms such as nausea, vomiting and diarrhea, occur 1-6
hours after eating and usually last a day or two. The
enterotoxins stimulate the vomit reflex center in the
CNS Bacterial endocarditis Most often caused by normal flora Staphylococci and
Streptococci of the mouth which travel into the blood and
produce vegetations and fibrin build-up on heart valves Acute infections can kill within days. Most
patients have subacute disease which manifests as
malaise, fever, arrhythmias and heart murmurs. Congestive
heart failure is the most common complication and cause
of death. Osteomyelitis S. aureus can travel via the blood and infect the
terminal vessels of long bones
Treatment of S. aureus infections
Prevention
Hygiene and regular hand-washing, particularly in a hospital setting , is the most effective measure against transmission of S. aureus. Isolation of patients with MRSA and barrier precautions such as gloves are required in hospitals.
Food-handlers are also required to regularly wash hands. Sick food handlers should be excluded from work. Proper cooking and refrigeration of foods is also required.
Menstruating women are advised to avoid regular use of super absorbent tampons because of the association with TSS.
Prophylactic antibiotics are sometimes given to dental patients with existing heart valve damage prior to invasive procedures in order to prevent bacterial endocarditis. The American Heart Association's guidelines on who should receive prophylactic antibiotics have been revised several times and also take into account the rising problem of drug resistance.
Streptococcus spp.
Cultural and morphological characteristics
Streptococcus refers to round cells arranged in chains. This genus tests catalase negative. Organisms can be classified according to specific carbohydrate surface antigens using the Lancefield system of serotyping. Among the most medically significant species are Group A streptococci which produce B-hemolysis on blood agar and show sensitivity to the antibiotic bacitracin. Among the most important of the Group A streptococci is the pathogen Streptococcus pyogenes.
Source of images: L. Tomalty, Dept of Microbiology, Queens University, Ontario, Canada.
Diseases caused by strains of Streptococci
Like S. aureus, certain streptococci have the ability to cause a wide variety of infections ranging from skin infections, pneumonia, septicemia and endocarditis. Many of the disease manifestations are produced by a wide variety of exoenzymes and exotoxins. This organism is protected against phagocytosis by a surface coating known as M protein.
Pharyngitis (Strep throat) Inhaling droplet nuclei from active cases or healthy
carriers A painful sore throat, swollen adenoids with
pus-containing lesions, fever and vomiting. Skin infections Impetigo is easily transmitted by skin contact and
handling objects such as toys and furniture Before antibiotics erysipelas occurred after the
introduction of hemolytic streptococci into wounds, cuts or
after surgery "Flesh-eating bacteria" can also enter via wounds, cuts
or be introduced into the tissue by injection drug use. Impetigo is a very contagious pyogenic infection
that usually involves both staphylococci and streptococci.
Most infections occur in young children and involve
crusty-oozing lesions about the nose and mouth Erysipelas means reddening of the skin. Organisms
multiply at margins of lesions and destroy tissue with
enzymes such as hyaluronidase. Cellulitis (destruction of solid tissue) &
fasciitis (destruction of muscle covering) can result
from invasive strains of streptococci which release tissue
invasive enzymes such as hyaluronidase and streptokinase Scarlet fever As for strep throat Caused by strains of S.pyogenes infected with a
toxin-encoding phage. Three toxins have been
identified. People who lack antibody against the toxins
develop a high fever and red rash on the body. "Strawberry
tongue" may also result. Rheumatic fever Follows strep throat in about 3% cases Usually occurs 2-3 weeks after strep throat has resolved.
Symptoms include fever, arthritis and a rash. Mitral valve
damage and endocarditis are common events. Antibodies formed
against certain streptococcal antigens can cross-react with
and damage heart tissue. A genetic predisposition to the disease is strongly
suspected Puerperal (childbed)fever S. pyogenes introduced into the blood during delivery This disease was common before antibiotics. The resulting
septicemia leads to fever, pelvic distention & pain, a
bloody vaginal discharge and death if untreated Bacterial endocarditis See notes for S. aureus See notes for S. aureus
Group B Streptococcal disease
This disease is caused by Streptococcus agalactiae and is the leading cause of sepsis and meningitis in newborns in the U.S. and Europe. Between 1-3 cases per 1000 births occur with a 50% mortality. Group B strep are common among vaginal flora and can be transmitted during delivery. A few days after birth the infant can develop fever, lethargy and difficulty breathing. Meningitis can occur weeks later. Pregnant women need to be tested for Group B strep in the third trimester and given ampicillin if colonized. Newborns of colonized mothers also need ampicillin for about a week.
Streptococcus pneumoniae
Cultural and morphological characteristics
S. pneumoniae appear as encapsulated, lancet-shaped diplococci. On blood agar they produce a-hemolysis and a greenish pigment appears around the colonies. In vitro the organisms are sensitive to the drug optochin and resistant to bacitracin.
Diseases caused by S. pneumoniae
In the U.S.A. more deaths result from pneumonia than any other infectious disease. Pneumonia can be caused by many different microbes. S. pneumoniae causes 95% of all cases of lobar pneumonia (affecting one or more of the 5 lobes of the lungs). Accumulations of fibrin and organisms block air spaces and pleurisy, inflammation of the lung lining, are common manifestations. If untreated there is a 30% mortality. When treated quickly there is a rapid response to antibiotics within 24 hours.
The disease is transmitted by respiratory droplets and the onset is abrupt. Symptoms include high fever, chest pain, profound fatigue, difficulty breathing, and productive cough.
This organism is also the most common cause of meningitis in adults, spreading from the lungs via the bloodstream. It can also cause acute otitis media
Prevention.
A vaccine called Pneumovax: protects against the 23 most common capsular strains of S. pneumoniae that are responsible for over 80% of pneumococcal pneumonia in the U.S. The vaccine is recommended for the elderly and for at risk populations such as immune-suppressed persons. This vaccine does not protect infants < 2 yrs.
A newer vaccine called Prevnar was licensed in 2000 and can be given to young children to prevent pneumonia and ear infections. It is costly: 4 doses cost around $230.
Treatment of Streptococcal infections
Penicillins have traditionally been the drug of choice for Streptococcal infections and especially for pneumococcal pneumonia. Cephalosporins and erythromycin can also be used for sensitive strains of S. pyogenes.