Infectious Disease Online
Pathology of Staphylococcal Infections
Staphylococcal infections are caused by species of the genus Staphylococcus.
These gram-positive cocci colonize the skin and anterior nasal vestibule of children and adults, and the umbilicus, stool and perineum of neonates.
Three species are pathogenic in humans : Staphylococcus aureus , Staphylococcus epidermidis and Staphylococcus saprophyticus.
About 20% to 40% of adults are nasal carriers of Staphylococcus aureus, and many become carriers while in the hospital, perhaps because medical personnel are more frequent carriers than the general population.
Large numbers of staphylococci are required for infection, and patients with more than 1000000 organisms per milliliter of nasal fluid tend to infect themselves and spread the staphylococci to others.
Most staphylococcal infections are caused by Staphylococcus aureus , which grows especially well on skin and mucous membranes but can infect any part of the body.
Staphylococcus aureus causes a wide variety of suppurative diseases, including among others abscesses of the skin (impetigo, boils, styes, carbuncles, breast abscesses, botryomycosis), abscesses of bone (osteomyelitis) and other deep organs, infections of burns and surgical and other wounds, infections of the upper and lower respiratory tracts (pharyngitis, bronchopneumonia , empyema), purulent arthritis, septicemia, acute endocarditis and meningitis.
Staphylococcus aureus releases several exotoxins : enterotoxins, (enteritis and food poisoning) ; exfoliative toxin (exfoliative skin disease); and pyrogenic toxin (toxic shock syndrome).
Staphylococcus epidermidis causes only minor skin lesions, except in patients who have surgically inserted prostheses or are immunodeficient.
Staphylococcus saprophyticus is responsible for bladder infections.
Staphylococcus aureus, named for its golden-yellow colonies on blood agar, produces coagulase (i.e., “coagulase-positive”).
Staphylococcus epidermidis and Staphylococcus saprophyticus, which form white colonies on blood agar (hence the former name Staphylococcus albus), are coagulase-negative.
Staphylococci are spherical and about 1 micrometer in diameter, a size that usually requires an oil immersion lens for identification in tissue sections.
In liquid culture medium as well in tissue, staphylococci grow in characteristic clusters, because they divide in successive perpendicular planes and daughter cells do not separate.
These clusters distinguish staphylococci from streptococci, which grow in chains.
Many strains of staphylococci have developed resistance to penicillin and other antibiotics.
Penicillin resistance is caused by plasmid-mediated production of penicillinase.
Infection of the Skin:
Staphylococcal infections cause a variety of cutaneous and systemic infections, including Impetigo, Furuncles (boils), subcutaneous abscess, staphylococcal scalded skin syndrome, Toxic Shock Syndrome and neonatal toxic shock syndrome-like exanthematous disease, in association with microbial virulence factors.
Staphylococcal scalded skin syndrome: The exfoliative toxin of S. aureus causes the “scalded skin syndrome”, which usually occurs in neonates, infants, and young children, typically in the aftermath of conjunctivitis or minor staphylococcal infection.
A painful, brick-red rash begins on the face, neck, axilla, and groin, and then becomes generalized.
The rash leads to blisters or bullae, and the upper dermis is shed in large sheets.
Breast abscesses usually arise within a few weeks after delivery, when staphylococci are transmitted from an infant with neonatal sepsis to the skin glands in the breasts of the nursing mother.
The disease may be precipitated by the stasis of milk after weaning or missed feeding.
Botryomycosis (a misnomer) is a chronic bacterial infection that may be caused by staphylococci (as well as by Streptococci ), E. coli, and other common bacteria).
Botryomycosis patients as an indurated fibrotic mass with draining sinuses and grains in a purulent exudates and in tissue sections.
Microcolonies of staphylococci in clusters within the grain are surrounded by an amorphous eosinophilic coating (“Splendore-Hoeppli phenomenon”).
Botryomycosis resists antibiotic therapy, probably because the fibrosis and compactness of the grains prevents adequate levels of drug from reaching the bacteria.
The lesion should be totally excised.
Abscesses of Bone (Osteomyelitis):
Acute staphylococcal osteomyelitis most commonly afflicts boys between 3 to 10 years of age, most of whom have a history of infection or trauma.
The bones of the legs are involved in most patients.
Many patients have an underlying bacteremia (S. aureus) with systemic symptoms.
Osteomyelitis may become chronic if not properly treated.
Adults after 50 years of age are more frequently afflicted with osteomyelitis of the vertebra.
The onset of localized back pain is usually abrupt, but may follow staphylococcal infection of the skin or urinary tract, prostatic surgery, infected abortion, puerperal infection, or a surgical procedure such as pinning a fracture.
Infections of Burns and Surgical Wound:
Burns and surgical wounds may become infected with S. aureus from the patient’s own nasal carriage or from medical personnel.
The appearance of visible pus in the wound depends on the interaction of bacteria, host factors, and foreign bodies.
Neonates, the elderly, the malnourished, and the obese all have increased susceptibility.
Infections of the upper and lower respiratory tract (Pharyngitis ; Bronchopneumonia and Empyema)
Staphylococcal infections of the respiratory tract most commonly occurs in infants less than 2 years of age, and especially in those under 2 months.
They usually occur in winter, when viral respiratory diseases are prevalent.
The child often has an underlying staphylococcal skin infection. Infection of the respiratory tract is mild at first, but suddenly worsens.
Characteristic features include fever and spasms of dry coughing, followed by marked tachypnea with expiratory grunting, sternal retraction, cyanosis, progressive lethargy, and shock.
There are ulcers of the upper airway and scattered foci of pneumonia.
Other common complications are pleural effusion, empyema, and pneumothorax.
Radiological examination of the chest show patchy infiltrates, which progress rapidly.
Gram-positive cocci are seen in aspirated tracheal or pleural fluid, which is often bloody.
In adults, staphylococcal pneumonia may follow viral influenza, a disease that destroyed the ciliated surface epithelium and leaves the bronchial surface vulnerable to secondary infections.
Patients with chronic lung disease and chronic heart disease (especially rheumatic valve disease) are also at increased risk for staphylococcal pneumonia.
Acute and Chronic Bacterial Arthritis:
S. aureus is the causative organism in half of all cases of septic arthritis. Most of those who have the disease are adults, 50 to 70 years old, and usually only a single joint is involved.
Rheumatoid arthritis and steroid therapy are common predisposing conditions.
The acute onset of staphylococcal arthritis is marked by severe, throbbing pain, often worse at night, which is accompanied by shaking chills and fever.
Acute staphylococcal arthritis may be confused with an acute episode of rheumatoid arthritis.
Septicemia with S. aureus occurs in patients with lowered resistance who are in the hospital for other diseases or conditions.
Some having underlying staphylococcal infections (for example, osteomyelitis or septic arthritis), some have had surgery (especially transurethral resection of the prostate), and some have infections from an indwelling intravenous catheter.
Staphylococcal septicemia is associated with the common symptoms of bacteremia, such as shaking chills and fever.
Miliary abscesses and staphylococcal endocarditis are serious complications.
Acute and subacute bacterial endocarditis are complications of septicemia caused by S. aureus (as well as by S. epidermidis).
Endocarditis may develop spontaneously on normal valves or on valves damaged by rheumatic fever.
It may also follow insertion of prosthetic valves or other intracardiac surgery.
Those with intravenous heroin addiction also have an increased risk of endocarditis from infection with S. aureus.
In addition to the symptoms of septicemia, a heart murmur is usual, with or without evidence of embolization to other organs.
Staphylococcal meningitis is a complication of surgical procedures on the central nervous system.
Infections of shunts in the brain may be caused by S. aureus or S. epidermidis.
Although staphylococcal meningitis is often not clinically evident, it may be found at autopsy in patients with septicemia or endocarditis.
Staphylococcal Food Poisoning:
Staphylococcal food poisoning is caused by the ingestion of preformed staphyloccal enterotoxin in prepared food.
This commonly involves food eaten in a restaurant (not industrially processed food), especially unrefrigerated meats, milk, or custard and other milk products.
The food (not the patient’s excreta) must be tested for staphylococci.
Food that contains more than 100000000 staphylococci per gram contains enough enterotoxin to cause food poisoning.
S. aureus has caused more than half of the food poisoning epidemics in which causative agent has been identified.
At least six enterotoxins are produced by some of the coagulase-positive strains of S. aureus, and enterotoxins are also produced by by a few coagulase-negative strains.
Enterotoxins are resistant to heat and withstand cooking for 20 to 60 minutes.
Usually, nausea and vomiting begin within a few hours of ingesting the toxin.
In some cases, however, diarrhea and abdominal discomfort are the only symptoms.
Patients with more severe food poisoning have bloody mucus in the vomitus and stools, as well as muscle cramps, headache, and sweating.
The acute phase commonly lasts 4 to 6 hours, and recovery is complete within 1 or 2 days.
Acute gastroenteritis is characterized by histologic changes in both stamach and small intestine.
Within 2 hours of the introduction of enterotoxin, there is a neutrophilic exudates in the stomach.
By 6 hours the gastric mucosal cells are depleted of mucus, and the mucosa is covered by a mucopurulent exudates.
The inflammatory reaction in the stomach subsides within 24 hours.
In the small intestine, by 4 hours there is focal degeneration of the epithelium of the villi, elongation of crypts, and infiltration of neutrophils in the lamina propria.
After 12 hours regression begins, and by 48 to 72 hours the mucosa appears normal.
Infections with S. Epidermidis:
S. epidermidis, an opportunistic pathogen, causes only minor skin lesions, except in patients undergoing surgery for insertion of prosthetic devices and patients with impaired immune systems.
In healthy persons, the organism usually resides on the skin of the axilla, head, nose, and limbs.
Infections with S. epidermidis are often associated with foreign bodies, such as prosthetic valves, shunts for cerebrospinal fluid, joint prostheses.
Prosthetic valvular endocarditis, for example, may be caused by contaminated coronary suction lines during the insertion of prosthetic valves, with subsequent infection of repaired areas of the heart and prosthetic valve.
Deep sternal wound infections may result, often in the first few weeks after the operation.
S. epidermidis can be the direct cause (without foreign body) of bladder infections, endocarditis, and other infections.
Strains of S. epidermidis are frequently resistant to penicillin and other antimicrobial agents, and infected prostheses and grafted vessels are often need to be replaced.
Infection with S. Saprophyticus:
S. saprophyticus resembles S. epidermidis, but biochemical assays and the pattern of drug resistance distinguish the two.
For reasons unknown, S. saprophyticus causes bladder infections, primarily in young women.
Strains of S. saprophyticus are sensitive to many antibiotics.