The Normal Flora

The normal flora is the population of microorganisms routinely found growing on the body of healthy individuals (figure 19.1). Microbes that typically inhabit body sites for extended periods are called the resident flora, whereas those that are only temporary are termed the transient flora. Many different species of microorganisms make up the normal flora, and they occur in large numbers. In fact, there are more bacteria in just one person's mouth than there are people in the world!

The Protective Role of the Normal Flora

The most significant contributions of the normal flora to the overall health of the human host include protection against potentially harmful microorganisms, and stimulation of the immune system. When organisms of the normal flora are killed or their growth suppressed, as can happen during treatment of the host with antibiotics, harmful organisms may colonize and cause disease. For example, oral administration of antibiotics can suppress normal intestinal flora, allowing the overgrowth of toxin-producing strains of Clostridium difficile, causing the disease antibiotic-associated colitis. ■ antibiotic-associated colitis, p. 601

As we discussed in chapter 15, the presence and multiplication of normal flora competitively excludes pathogens by several different mechanisms. These include covering binding sites that might otherwise be used for attachment, consuming available nutrients, and producing compounds that are toxic to other bacteria. ■ normal flora, p. 375

Mouth

Streptococcus species Fusobacterium species Actinomyces species Leptotrichia species Veillonella species

Skin

Staphylococcus epidermidis Propionibacterium acnes Pityrosporum ovale

Vagina

Lactobacillus species Streptococcus species Candida albicans Gardnerella vaginalis

Urethra

Streptococcus species Mycobacterium species Escherichia coli Bacteroides species

Figure 19.1 Normal Flora Location of many of the organisms that are part of the normal flora of the male and the female human body.

Throat

Streptococcus species Branhamella catarrhalis Corynebacterium species Haemophilus species Neisseria species Mycoplasma species

Figure 19.1 Normal Flora Location of many of the organisms that are part of the normal flora of the male and the female human body.

What Normal Flora And Transient Flora

Mouth

Streptococcus species Fusobacterium species Actinomyces species Leptotrichia species Veillonella species

Skin

Staphylococcus epidermidis Propionibacterium acnes Pityrosporum ovale

Vagina

Lactobacillus species Streptococcus species Candida albicans Gardnerella vaginalis

Urethra

Streptococcus species Mycobacterium species Escherichia coli Bacteroides species

Large intestine

Bacteroides fragilis Escherichia coli Proteus mirabilis Klebsiella species Lactobacillus species Streptococcus species Candida albicans Clostridium species Pseudomonas species Enterococcus species

462 Chapter 19 Host-Microbe Interactions

The normal flora also plays an important role in priming the adaptive immune system. The response mounted against members of the normal flora, which routinely breach the body's anatomical barriers in small numbers, may cross-react with pathogens that could be encountered later. The importance of the normal flora in the development of immune responses is shown in mice reared in a microbe-free environment. These animals lack a normal flora and have greatly underdeveloped mucosal-associated lymphoid tissues (MALT), the regions under mucous membranes where lymphocytes congregate waiting for invading microbes. ■ MALT, p. 397

The Dynamic Nature of the Normal Flora

A healthy human fetus is sterile until the protective membrane that surrounds it ruptures as a prelude to birth. During the passage through the birth canal, the baby is exposed to a variety of microbes that take up residence on its skin and in its gastrointestinal tract. Various microorganisms in food or on other humans and in the environment soon also become established as residents on the newborn.

Once established, the composition of the normal flora is dynamic. At any one time the makeup of this complex ecosystem represents a balance of many forces that may dramatically or discreetly alter the bacterial population's quantity and composition. Changes occur in response to physiological variations within the host, such as hormonal changes, and as a direct result of the activities of the human host, such as the type of food consumed. Further, each member of the ecosystem is influenced by the presence and condition of others. Most antibiotics will kill or inhibit not only pathogens, but also some of the normal flora, allowing unwanted organisms to proliferate. For example, the Lactobacillus species that predominate in the vagina of mature females normally suppress growth of the yeast Candida albicans. During intensive, unrelated, treatment with certain antibiotics, however, the normal bacterial flora may be inhibited, allowing the fungi to overgrow and cause disease. In hospitalized patients, the suppressed normal flora may be undesirably replaced by antibiotic-resistant strains of genera such as Pseudomonas or Staphylococcus, which can cause serious infection.

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Responses

  • belinda
    What is normal flora and transient flora?
    5 years ago
  • willie
    What happens to organisms when normal flora is replaced with transient flora?
    5 years ago
  • eliisa
    What is the causative agent of microflora?
    2 years ago
  • Ruby
    What is multiplication of normal flora?
    7 months ago
  • jefferson
    What can cause suppression of oral flora?
    5 months ago

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