Page 38 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com Important Organisms 1 Less Important Organisms 2 CHAPTER 6 Normal Flora 27 mebooksfree.com
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TABLE 6–2 Medically Important Members of the Normal Flora
Location

Staphylococcus aureus, Corynebacterium (diphtheroids), various streptococci,
Skin
Staphylococcus epidermidis
Pseudomonas aeruginosa, anaerobes (e.g., Propionibacterium), yeasts (e.g.,
Candida albicans)
Nose
Viridans streptococci
Various streptococci, Eikenella corrodens
Mouth
Prevotella intermedia, Porphyromonas gingivalis
Streptococcus mutans
Dental plaque S. aureus 3 S. epidermidis, Corynebacterium (diphtheroids), various streptococci
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mebooksfree.com mebooksfree.com Viridans streptococci 3 Various streptococci (including Streptococcus pyogenes and Streptococcus mebooksfree.com

Gingival crevices
Various anaerobes (e.g.,
Bacteroides, Fusobacterium,
streptococci, Actinomyces)
Throat
pneumoniae), Neisseria species, Haemophilus influenzae, S. epidermidis
Bacteroides fragilis, Escherichia coli
Colon
Bifidobacterium, Eubacterium, Fusobacterium, Lactobacillus, various aerobic
gram-negative rods, Enterococcus faecalis and other streptococci, Clostridium
Various streptococci, various gram-negative rods. B. fragilis, Corynebacterium
Vagina
Lactobacillus, E. coli, group B
3
streptococci
(diphtheroids), C. albicans

Urethra
3
gram-negative rods (e.g., E. coli)
2 1 3 Organisms that are medically significant or present in large numbers. S. epidermidis, Corynebacterium (diphtheroids), various streptococci, various
Organisms that are less medically significant or present in smaller numbers.
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These organisms are not part of the normal flora in this location but are important colonizers.
The members of the normal flora play a role both in the
maintenance of health and in the causation of disease in
bacteria, but yeasts and protozoa are also found in large
numbers. Using sophisticated molecular techniques, many
three significant ways:
previously unknown bacteria have been identified. Routine
(1) They can cause disease, especially in immunocom-
cultures on bacteriological media typically reveal only a
promised and debilitated individuals. Although these
small subset of the existing resident organisms.
organisms are nonpathogens in their usual anatomic loca-
13
It is estimated that the adult human contains 10 cells
tion, they can be pathogens in other parts of the body.
(2) They constitute a protective host defense mechanism.
14
is approximately 10 , that is, there are 10 times more
The nonpathogenic resident bacteria occupy attachment
microbial cells than human cells. The largest and most
sites on the skin and mucosa that can interfere with coloniza- whereas the number of microbes in the human microbiome
complex microbial population resides in the colon. Within
tion by pathogenic bacteria. The ability of members of the
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the colon, the two largest phyla of bacteria are the Fir-
normal flora to limit the growth of pathogens is called colo-
micutes (64%) and the Bacteroidetes (23%). The Firmicutes
nization resistance. If the normal flora is suppressed, patho-
gens may grow and cause disease. For example, antibiotics
tridium and Faecalibacterium are prominent organisms.
can reduce the normal colonic flora that allows Clostridium
The Bacteroidetes are gram-negative rods and the genera,
difficile, which is resistant to the antibiotics, to overgrow and
Bacteroides and Prevotella are important members. Species
cause pseudomembranous colitis.
of Proteobacteria (gram-negative rods such as Escherichia
(3) They may serve a nutritional function. The intesti-
and Salmonella) and Actinobacteria (gram-positive rods
nal bacteria produce several B vitamins and vitamin K.
such as Actinomyces) make up a large percentage of the
Poorly nourished people who are treated with oral antibi-
remainder.
otics can have vitamin deficiencies as a result of the reduc-
There is mounting evidence that the organisms in the
tion in the normal flora. However, since germ-free animals
microbiome play an important role in several body func-
are well-nourished, the normal flora is not essential for
tions and diseases, such as weight control (obesity), inflam-
proper nutrition.
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resistance to infectious disease.
The effect on obesity is revealed by studies involving the
THE HUMAN MICROBIOME
transfer of fecal bacteria between strains of inbred mice.
For example, fecal bacteria from obese mice transplanted
The human microbiome is the term used to describe the
thousands of microbes (“microbiota”) located on the skin,
into germ-free strains of nonobese mice resulted in the
on mucosal surfaces, and within the lumen of the
nonobese mice becoming obese. It appears that the fecal
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