Gram positive cocci

Gram-Positive Cocci: Introduction

There are two medically important genera of gram-positive cocci: Staphylococcus and Streptococcus. Two of the most important human pathogens, Staphylococcus aureus and Streptococcus pyogenes, are described in this chapter. Staphylococci and streptococci are nonmotile and do not form spores.

Both staphylococci and streptococci are gram-positive cocci, but they are distinguished by two main criteria:

Microscopically, staphylococci appear in grapelike clusters, whereas streptococci are in chains.

Biochemically, staphylococci produce catalase (i.e., they degrade hydrogen peroxide), whereas streptococci do not.

Staphylococcus

Diseases

Staphylococcus aureus causes abscesses, various pyogenic infections (e.g., endocarditis, septic arthritis, and osteomyelitis), food poisoning, scalded skin syndrome, and toxic shock syndrome. It is one of the most common causes of hospital-acquired pneumonia, septicemia, and surgical-wound infections. It is an important cause of skin infections, such as folliculitis, cellulitis, and impetigo. It is the most common cause of bacterial conjunctivitis.

Staphylococcus epidermidis can cause endocarditis and prosthetic joint infections. Staphylococcus saprophyticus causes urinary tract infections. Kawasaki syndrome is a disease of unknown etiology that may be caused by certain strains of Sta. aureus.

Important Properties

Staphylococci are spherical gram-positive cocci arranged in irregular grapelike clusters (Figure). All staphylococci produce catalase, whereas no streptococci do (catalase degrades H2O2 into O2 and H2O). Catalase is an important virulence factor because H2O2 is microbicidal and its degradation limits the ability of neutrophils to kill.

Three species of staphylococci are human pathogens: Sta. aureus, Sta. epidermidis, and Sta. saprophyticus (Table 1). Of the three, Sta. aureus is by far the most important. Sta. aureus is distinguished from the others primarily by coagulase production. Coagulase is an enzyme that causes plasma to clot by activating prothrombin to form thrombin. Thrombin then catalyzes the activation of fibrinogen to form the fibrin clot. Sta. epidermidis and Sta. saprophyticus are often referred to as coagulase-negative staphylococci.

Sta. aureus produces a carotenoid pigment called staphyloxanthin, which imparts a golden color to its colonies. This pigment enhances the pathogenicity of the organism by inactivating the microbicidal effect of superoxides and other reactive oxygen species within neutrophils. Sta. epidermidis does not synthesize this pigment and produces white colonies. The virulence of Sta. epidermidis is significantly less than that of Sta. aureus. Two other characteristics further distinguish these species, namely, Sta. aureus usually ferments mannitol and hemolyzes red blood cells, whereas the others do not. More than 90% of Sta. aureus strains contain plasmids that encode -lactamase, the enzyme that degrades many, but not all, penicillins. Some strains of Sta. aureus are resistant to the -lactamase–resistant penicillins, such as methicillin and nafcillin, by virtue of changes in the penicillin-binding protein (PBP) in their cell membrane. Genes on the bacterial chromosome called mecA genes encode these altered PBPs.

These strains are commonly known as methicillin-resistant Sta. aureus (MRSA) or nafcillin-resistant Sta. aureus (NRSA). MRSA currently accounts for more than 50% of Sta. aureus strains isolated from hospital patients in the United States. The most common strain of MRSA in the United States is the "USA300" strain.

Strains of Sta. aureus with intermediate resistance to vancomycin (VISA) and with full resistance to vancomycin (VRSA) have also been detected. The cassette of genes that encodes vancomycin resistance in Sta. aureus is the same as the cassette that provides vancomycin resistance in enterococci. These genes are located in a transposon on a plasmid and encode the enzymes that substitute D-lactate for D-alanine in the peptidoglycan.

Sta. aureus has several important cell wall components and antigens:

• 1. Protein A is the major protein in the cell wall. It is an important virulence factor because it binds to the Fc portion of IgG at the complement-binding site, thereby preventing the activation of complement. As a consequence, no C3b is produced, and the opsonization and phagocytosis of the organisms are greatly reduced. Protein A is used in certain tests in the clinical laboratory because it binds to IgG and forms a "coagglutinate" with antigen–antibody complexes. The coagulase-negative staphylococci do not produce protein A.

  2. Teichoic acids are polymers of ribitol phosphate. They mediate adherence of the staphylococci to mucosal cells and play a role in the induction of septic shock.

  3. Polysaccharide capsule is also an important virulence factor. There are 11 serotypes based on the antigenicity of the capsular polysaccharide, but types 5 and 8 cause 85% of infections. Some strains of Sta. aureus are coated with a small amount of polysaccharide capsule called a microcapsule. The capsule is poorly immunogenic, which has made producing an effective vaccine difficult.

  4. Surface receptors for specific staphylococcal bacteriophages permit the "phage typing" of strains for epidemiologic purposes. Teichoic acids make up part of these receptors.

  5. The peptidoglycan of Sta. aureus has endotoxin-like properties, i.e., it can stimulate macrophages to produce cytokines and can activate the complement and coagulation cascades. This explains the ability of Sta. aureus to cause the clinical findings of septic shock yet not possess endotoxin.

Transmission

Humans are the reservoir for staphylococci. The nose is the main site of colonization of Sta. aureus and approximately 30% of people are colonized at any one time. People who are chronic carriers of Sta. aureus in their nose have an increased risk of skin infections caused by Sta. aureus.

The skin, especially of hospital personnel and patients, is also a common site of Sta. aureus colonization. Hand contact is an important mode of transmission and handwashing decreases transmission.

Sta. aureus is also found in the vagina of approximately 5% of women, which predisposes them to toxic shock syndrome. Additional sources of staphylococcal infection are shedding from human lesions and fomites such as towels and clothing contaminated by these lesions.

Disease caused by Sta. aureus is favored by a heavily contaminated environment (e.g., family members with boils) and a compromised immune system. Reduced humoral immunity, including low levels of antibody, complement, or neutrophils, especially predisposes to staphylococcal infections. Diabetes and intravenous drug use predispose to infections by Sta. aureus. Patients with chronic granulomatous disease (CGD), a disease characterized by a defect in the ability of neutrophils to kill bacteria, are especially prone to Sta. aureus infections (see Chapter 68).

Sta. epidermidis is found primarily on the human skin and can enter the blood stream at the site of intravenous catheters that penetrate through the skin. Sta. saprophyticus is found primarily on the mucosa of the genital tract in young women and from that site can ascend into the urinary bladder to cause urinary tract infections.

Pathogenesis

Staphylococcus aureus

Sta. aureus causes disease both by producing toxins and by inducing pyogenic inflammation. The typical lesion of Sta. aureus infection is an abscess. Abscesses undergo central necrosis and usually drain to the outside (e.g., furuncles and boils), but organisms may disseminate via the bloodstream as well. Foreign bodies, such as sutures and intravenous catheters, are important predisposing factors to infection by Sta. aureus.

Several important toxins and enzymes are produced by Sta. aureus. The three clinically important exotoxins are enterotoxin, toxic shock syndrome toxin, and exfoliatin.

• 1. Enterotoxin causes food poisoning characterized by prominent vomiting and watery, nonbloody diarrhea. It acts as a superantigen within the gastrointestinal tract to stimulate the release of large amounts of interleukin-1 (IL-1) and interleukin-2 (IL-2) from macrophages and helper T cells, respectively. The prominent vomiting appears to be caused by cytokines released from the lymphoid cells, which stimulate the enteric nervous system to activate the vomiting center in the brain. Enterotoxin is fairly heat-resistant and is therefore usually not inactivated by brief cooking. It is resistant to stomach acid and to enzymes in the stomach and jejunum. There are six immunologic types of enterotoxin, types A–F.

  2. Toxic shock syndrome toxin (TSST) causes toxic shock, especially in tampon-using menstruating women or in individuals with wound infections. Toxic shock also occurs in patients with nasal packing used to stop bleeding from the nose. TSST is produced locally by Sta. aureus in the vagina, nose, or other infected site. The toxin enters the bloodstream, causing a toxemia. Blood cultures typically do not grow Sta. aureus.

     1. TSST is a superantigen and causes toxic shock by stimulating the release of large amounts of IL-1, IL-2, and tumor necrosis factor (TNF) (see the discussions of exotoxins in Chapter 7 and superantigens in Chapter 58). Approximately 5% to 25% of isolates of Sta. aureus carry the gene for TSST. Toxic shock occurs in people who do not have antibody against TSST.

  3. Exfoliatin causes "scalded skin" syndrome in young children. It is "epidermolytic" and acts as a protease that cleaves desmoglein in desmosomes, leading to the separation of the epidermis at the granular cell layer.

  4. Several toxins can kill leukocytes (leukocidins) and cause necrosis of tissues in vivo. Of these, one of the most important is alpha toxin, which causes marked necrosis of the skin and hemolysis. The cytotoxic effect of alpha toxin is attributed to the formation of holes in the cell membrane and the consequent loss of low-molecular-weight substances from the damaged cell.

     1. P-V leukocidin, a second important toxin, is a pore-forming toxin that kills cells, especially white blood cells, by damaging cell membranes. The two subunits of the toxin assemble in the cell membrane to form a pore through which cell contents leak out. The gene encoding P-V leukocidin is located on a lysogenic phage.

     2. The importance of P-V leukocidin as a virulence factor is indicated by the severe skin and soft tissue infection caused by MRSA strains that produce this leukocidin. A severe necrotizing pneumonia is also caused by strains of Sta. aureus that produce P-V leukocidin. Approximately 2% of clinical isolates of Sta. aureus produce P-V leukocidin.

  5. The enzymes include coagulase, fibrinolysin, hyaluro-nidase, proteases, nucleases, and lipases. Coagulase, by clotting plasma, serves to wall off the infected site, thereby retarding the migration of neutrophils into the site. Staphylokinase is a fibrinolysin that can lyse thrombi.

Staphylococcus epidermidis & Staphylococcus saprophyticus

Unlike Sta. aureus, these two coagulase-negative staphylococci do not produce exotoxins. Thus, they do not cause food poisoning or toxic shock syndrome. They do, however, cause pyogenic infections. For example, Sta. epidermidis is a prominent cause of pyogenic infections on prosthetic implants such as heart valves and hip joints.

Clinical Findings

The important clinical manifestations caused by Sta. aureus can be divided into two groups: pyogenic and toxin-mediated (Table 15–2). Sta. aureus is a major cause of skin, soft tissue, bone, joint, lung, heart, and kidney infections. In the following list, the first seven are pyogenic in origin, whereas the last three are toxin-mediated.