(1) The staphylococci and corynebacteria occur at
every site listed. Staphylococcus
epidermidis is highly adapted to the diverse environments of its
human host. S. aureus is a potential pathogen. It is a leading
cause of bacterial disease in humans. It can be transmitted from the
nasal membranes of an asymptomatic carrier to a susceptible host.
S. epidermidis. Scanning EM. CDC.
(2) Many of the normal flora are either
pathogens, The asterisks indicate members of the normal flora a that
be considered major pathogens of humans.
S. aureus. Gram stain.
(3) Streptococcus mutans is the primary
plaque formation and initiation of dental caries. Viewed as an
infection, dental disease is one of the most prevalent and costly
diseases in the United States.
Streptococcus mutans. Gram stain. CDC
(4) Enterococcus faecalis was formerly
classified as Streptococcus
faecalis. The bacterium is such a regular a component of the
flora, that many European countries use it as the standard indicator of
fecal pollution, in the same way we use E. coli in the
In recent years, Enterococcus faecalis has emerged as a
antibiotic-resistant, nosocomial pathogen.
Vancomycin Resistant Enterococcus faecalis. Scanning E.M. CDC
(5) Streptococcus pneumoniae is present in
tract of about half the population. If it invades the lower
tract it can cause pneumonia. Streptococcus pneumoniae
95 percent of all bacterial pneumonia.
Streptococcus pneumoniae. Direct fluorescent antibody stain.
(6) Streptococcus pyogenes refers to the
streptococci. Streptococci cause tonsillitis (strep throat), pneumonia,
endocarditis. Some streptococcal diseases can lead to rheumatic fever
or nephritis which can damage the heart and kidney.
Streptococcus pyogenes. Gram stain.
(7) Neisseria and other Gram-negative
frequent inhabitants of the upper respiratory tract, mainly the
meningitidis, an important cause of bacterial meningitis, can
as well, until the host can develop active immunity against the
Neisseria meningitidis. Gram stain.
(8) While E. coli is a consistent resident
of the small
many other enteric bacteria may reside here as well, including Klebsiella,
Enterobacter and Citrobacter. Some strains of E.
pathogens that cause intestinal infections, urinary tract infections
E. coli. Scanning E.M. Shirley Owens. Center for Electron
Michigan State University.
(9) Pseudomonas aeruginosa is the
pathogen of humans that can invade virtually any tissue. It is a
leading cause of hospital-acquired (nosocomial) Gram-negative
but its source is often exogenous (from outside the host).
Colonies of Pseudomonas aeruginosa growing on an agar plate.
The most virulent Pseudomonas species produce mucoid colonies and green
pigments such as this isolate.
(10) Haemophilus influenzae is a frequent
viral influenza, and was named accordingly. The bacterium was the
leading cause of meningitis in infants and children until the recent
of the Hflu type B vaccine.
Haemophilus influenzae. Gram stain.
(11) The greatest number of bacteria are found in
tract, specifically the colon and the most prevalent bacteria are the Bacteroides,
a group of Gram-negative, anaerobic, non-sporeforming bacteria.
have been implicated in the initiation colitis and colon cancer.
Bacteroides fragilis. Gram stain.
(12) Bifidobacteria are Gram-positive,
acid bacteria. They have been described as "friendly" bacteria in the
intestine of humans. Bifidobacterium bifidum is the predominant
bacterial species in the intestine of breast-fed infants, where it
presumably prevents colonization by potential pathogens. These bacteria
are sometimes used in the manufacture of yogurts and are frequently
incorporated into probiotics.
Bifidobacterium bifidum. Gram stain
(13) Lactobacilli in the oral cavity
probably contribute to
formation that leads to dental caries. Lactobacillus
colonizes the vaginal epithelium during child-bearing years and
the low pH that inhibits the growth of pathogens.
Lactobacillus species and a vaginal squaemous epithelial cell.
(14) There are numerous species of Clostridium
the bowel. Clostridium perfringens is commonly isolated
feces. Clostridium difficile may colonize the bowel and
"antibiotic-induced diarrhea" or pseudomembranous colitis.
Clostridium perfringens. Gram stain.
(15) Clostridium tetani is included in the
table as an
of a bacterium that is "transiently associated" with humans as a
of the normal flora. The bacterium can be isolated from feces in
0 - 25 percent of the population. The endospores are probably
ingested with food and water, and the bacterium does not colonize the
Clostridium tetani. Gram stain.
(16) The corynebacteria, and certain related
are consistent skin flora. Some have been implicated as a cause
acne. Corynebacterium diphtheriae, the agent of
was considered a member of the normal flora before the widespread use
the diphtheria toxoid, which is used to immunize against the disease.
Corynebacterium diphtheriae. No longer a part of the normal flora.