Chapter Checkpoints
These are some important ideas you are learning in Chapter 18:

General Characteristics of the Staphylococci
The genus Staphylococcus contains 31 species. Most of these are human commensals, but S. aureus, S. epidermidis, and S. saprophyticus are pathogenic.

All species in the genus Staphylococcus are gram-positive facultative anaerobes that tolerate extremes of temperature, osmotic pressure, and drying. All are catalase positive.

Coagulase-positive staphylococci are by definition S. aureus, the most resistant of all non-spore- forming pathogenic bacteria.

S. aureus produces an impressive array of virulence factors which enable it to resist phagocytosis, destroy host tissue, and invade the blood. These include coagulase, hemolysins, leukocidins, enzymes which digest host tissue, and toxins which attack the epidermis, vascular smooth muscle, and the intestinal epithelium. These virulence factors enable S.aureus to cause both local and systemic infections.

Abscesses are localized staphylococcal infections. They occur predominantly in hair follicles, wounds, and bone marrow.

S. aureus causes disease only when the host is weakened by poor health, injury, disease, surgery, or immunodeficiency.

S. epidermidis and S. saprophyticus are coagulase negative staphylococci which cause opportunistic infections. S. epidermidis causes nosocomial infections and S. saprophyticus causes urinary tract infections.

Overuse and inappropriate prescription of anti-microbials have selected for multiple drug-resistant strains of S. aureus in hospitals, causing serious infections in susceptible patients. All S. aureus isolates should therefore be checked for antimicrobial sensitivity so an effective therapeutic is prescribed.

Consistent practice of universal precautions by ALL hospital staff is necessary to prevent nosocomial staphylococcal infections.

General Characteristics of the Streptococci and Related Genera
Streptococci are gram-positive cocci arranged in chains and pairs. They are facultative anaerobes which do not survive well outside the host. The genus includes both harmless commensals and formidable pathogens. All Streptococcus species are catalase negative, which distinguishes them from the Staphylococci.

Streptococci are classified immunologically by their cell wall antigens (Lancefield groups) and also by their hemolysis of red blood cells. Clinical identification of Streptococcus species is based on the sensitivity to bacitracin, presence of the CAMP reaction, and other biochemical tests.

Streptococci which can completely hemolyze red blood cells are termed "beta-hemolytic.'' This indicates the presence of hemolysins. Lancefield groups A, B, C, G, and some members of Group D are all beta-hemolytic. S. pyogenes is a Group A beta-hemolytic pathogen. It is extremely pathogenic because of its many virulence factors such as M-protein, tissue-digesting enzymes, streptolysins S and O, which attack leukocytes, kidney, and heart muscle. S. pyogenes also triggers type III hypersensitivity reactions, i.e., rheumatic fever and acute glomerulonephritis after the initial infection subsides.

Local infections caused by S. pyogenes include the skin infections impetigo and erysipelas, as well as pharyngitis ("strep throat'').

Alpha hemolytic streptococci are found in several Lancefield groups. Alpha-hemolysis is caused by a beta toxin which partially clears red blood cells in blood agar cultures. The Group B pathogens S. agalactiae is a common opportunist agent of wound, skin, and neonatal infections.

The Group D streptococcus Enterococcus faecalis causes opportunist infections of wounds, blood, endocardium, as will as the urinary and gastrointestinal tracts.

Streptococci from Groups C and G, usually animal flora, are increasingly isolated in pharyngitis, AGN, and bacteremias in immunocompromised patients.

Alpha-hemolytic streptococci of the viridans group cause dental caries as well as subacute bacterial endocarditis following dental surgery or injury to the oral mucosa.

Alpha-hemolytic S. pneumoniae causes 80% of all bacterial pneumonias. Young children, the elderly, and immunocompromised individuals are particularly susceptible. However, immunization against S. pneumoniae gives effective protection to these groups.

Most streptococcal infections can be cured with Penicillin G, except for the enterococci. They require combination drug therapy because of developing resistance to anti-microbials.

The Family Neisseriaceae: Gram-Negative Cocci
Neisseria are fastidious, non-motile, gram-negative, coffee bean-shaped cocci that are obligate anaerobes and live as commensals in the mucous membranes of mammals. The two significant pathogens in this group are Neisseria gonorrhoeae and Neisseria meningitidis.

N. gonorrhoeae is the causative agent of gonorrhea, a sexually transmitted disease that causes infections of the male and female reproductive tracts. Genital gonorrhea in males causes urethritis and painful urination. Genital gonorrhea in females can lead to PID and ectopic pregnancies. It also causes sterility, proctitis, and pharyngitis in the sexually active, as well as gonococcal eye infections of the newborn.

Gonorrhea is a reportable STD because it is one of the top five STDs worldwide. It is spread by direct contact with infected carriers, who are often asymptomatic. Humans are the only reservoirs. It is especially prevalent in 18?24 year olds. Gonorrhea is treated by combined drug therapy because of widespread resistance to penicillin and frequent concurrent infection by other STD agents, particularly chlamydia.

Virulence factors of N. gonorrhoeae include pili, a capsule, and a protease which inactivates IgA.

N. meningitidis is the causative agent of epidemic cerebrospinal meningitis. It is transmitted by respiratory secretions or droplets from infected carriers.

Meningococcus virulence factors include LPS endotoxin, IgA protease, pili, and a capsule. Toxins cause cardiac failure, vascular collapse, and clotting disorders. Immunization is available to protect at-risk groups.

Pathogenic Neisseriaceae are differentiated from nonpathogenic species by oxidase production.

Other Gram-Negative Cocci and Coccobacilli
Other Neisseriaceae implicated in infectious disease are Branhamella and Acinetobacter. Most Neisseriaceae, such as Moraxella sp., are harmless commensals or saprobes, but Branhamella catarrhalis causes opportunist infections in immunocompromised individuals. Acinetobacter is a common agent in soil and water, and is increasingly isolated in nosocomial infections.

Return to Ch. 18 Activities lOnline Learning Center