Supplemental Lecture (98/05/09 update) by Stephen T. Abedon (firstname.lastname@example.org)
- Chapter title: Normal Flora
- A list of vocabulary words is found toward the end of this document
- Normal flora are bacteria, fungi, and protozoa that live on or within the bodies of animals and plants. Normal flora, by definition, do not cause disease in healthy individuals. Instead, they are commensalists or mutualists with regard to the host. That is, in addition to basically not harming the host, they can even do some good. One example of the benefits normal flora present to their host is something called microbial antagonism.
- Symbiosis [symbiont, symbiotic]
- The intimate, other than short-term interaction of one organism with another.
- Question of harm:
- Symbioses come in a variety of kinds differentiated by the degree to which one of the organisms is helped or harmed by the other:
- Typically, in a symbioses at least one of the organisms involved is gaining something from the interaction.
- For microorganisms, it would be the host that is harmed and the microorganism that is doing the harming.
- Many of the Normal flora neither hurt, harm, nor benefit the carrying host.
- Such symbionts are called commensals.
- Their interaction is termed commensalism.
- Mutualisms are symbiotic interactions between two organisms in which both organisms benefit.
- One of the most important mutualistic roles of normal flora is as microbial antagonists.
Normal flora [normal microbiota]
- With parasitism the host is harmed while the symbiont (i.e., the parasite) gains.
All found externally:
- Not disease-causing:
- Normal flora are those not-typically-disease-causing microorganisms normally found in and on healthy individuals.
- Also known as normal microbiota.
- Very abundant:
- Normal flora are extremely abundant in terms of absolute numbers.
- A normal human has approximately 1013 body cells and 1014 individual normal flora!
- However, microorganisms also tend to be very small, bacteria especially are much smaller than are our own cells.
Normal flora are found mostly:
Note that this list basically includes all of the body surfaces exposed to the external environment.
- on the skin
- in the eyes
- in the nose
- in the mouth
- in the upper throat
- in the lower urethra
- in the lower intestine
- especially in the large intestine
Normal flora are symbionts
- Transient microbiota are members of the normal flora that are not always present or are present for only a few days, weeks, or months before disappearing.
Microbial antagonism [microbial competition]
- Normal flora tend to be commensal or mutual symbionts adapted to the special conditions found in various body locations.
- Injuring avoidance:
- This tendency to not cause harm occurs at least because normal flora tend to avoid directly injuring the host.
- Alternative, more adaptationally sound means of viewing this behavior include:
- those normal flora that survive are only those not directly injuring the host, or
- damaging the host can result in a net loss in evolutionary fitness by those organisms doing the damaging (a view very consistent with the idea that pathogenic microorganisms tend to be ultimately either killed by the otherwise healthy host, or end up killing the host), or
- normal flora typically fail to gain from damaging their host even over the short term
Acquisition of normal flora
- Colonization inhibition:
- A process by which pathogenic microorganisms are inhibited by normal flora from colonizing healthy organisms.
- Mechanisms by which this inhibition occurs includes:
- competing with pathogenic microorganisms for nutrients
- competing with pathogenic microorganisms for space
- producing toxins that are harmful to some pathogenic microorganisms
- The presence of normal flora can affect the environment in which they live such that the environments physical characteristics become less suitable to colonization by pathogenic microorganisms.
- The lactobacilli which colonies the vagina lower the pH of that environment thus making an inhospitable habitat to numerous other would-be colonizers, some of which could be deemed pathogenic.
- Microbial antagonism, not necessarily mutualism?
- Whether a given example of symbiosis is considered mutualism or commensalism seems to have a lot to do with how unique is the microorganism occupying a niche on a body.
- For example, if the action of the microorganism is to simply take up space which might be taken up by any kind of microorganism, pathogenic or not, then the relationship is considered closer to commensal. However, if the microorganism either is unique in its ability to occupy that space/niche or the microorganism actively inhibits pathogenic microorganisms, then the relationship is considered to be more mutual.
- Personally, I consider such considerations to be fairly irrelevant semantic rantings. More important is that what a host might get out of its symbiotic relationship with a not necessarily unique microorganism is some level of protection against colonization by pathogenic microorganisms.
Normal flora, large intestine
- Environmental acquisition:
- In utero mammals are as sterile as any internal organ, which is to say, nearly if not entirely so.
- It is through contact with adults, older children, and the rest of their environment that newborns acquire all their normal flora.
Normal flora, mouth
- Huge, anoxic, environment:
- Kept anoxic by facultative anaerobes, the large intestine supports huge anaerobic populations.
- The greatest numbers of normal flora found on or in an animal are found in the large intestine.
- briefly, the colonization of the large intestine occurs in stages including. That is,
- during a vaginal birth, the newborn comes into contact with the lactobacilli of the mother. These lactobacilli are the first colonizers of the newborn's intestine.
- Escherichia coli, another lactose fermenter, soon follows.
- The newborn does not start acquiring the complex intestinal flora of the parent until they begin to feed on something other than milk.
- Where do the stage 2 and 3 organisms come from? Basically from oral exposure to feces.
- Note that many of the stage 3 organisms are strict anaerobes. They can coloninize only after the colonization of sufficient facultative organisms such that oxygen concentrations are maintained at drastically reduced levels.
Normal flora, skin
- Moist, warm, and awash with food, the mouth supports a diverse flora.
Examples of human normal flora
- Microbes able to survive on the skin must be able to withstand the antimicrobial properties associated with sweat and oil secretions.
- Actinomyces spp.
- aerobic micrococci
- Bacteroides spp.
- Bifidobacterium spp.
- Candida spp.
- Clostridium spp.
- Citrobacter spp.
- Corynebacterium spp.
- Corynebacterium xerosis
- Enterobacter spp.
- Enterococcus spp.
- Escherichia coli
- Fusobacterium spp.
- Hemophilus spp.
- Klebsiella spp.
- Lactobacillus spp.
- Neisseria spp.
- Pityrosporum spp.
- Propionibacterium acnes
- Proteus spp.
- Pseudomonas spp.
- Shigella spp.
- Staphylococcus spp.
- Staphylococcus aureus
- Staphylococcus epidermidis
- Streptococcus spp.
- Streptococcus pneumoniae
- Treponema spp.
- Trichomonas vaginalis
- Microbial antagonism
- Normal flora
- Normal flora are symbionts
- Transient microbiota
Practice question answers
- A colonic microorganism which bores into its host and by doing so does it damage, competes with its host for nutrients, and is the sole supplier of an organic growth factor without which the host could not survive. This symbiotic relationship is an example of (chose the best answer): [PEEK]
- an effector of microbial antagonism
- all of the above
- none of the above
- A bacteria is attached to the intestinal wall via its pili, causes its host no harm, and secretes an antibacterial poison which limits colonization of the intestine by other, unrelated bacteria. With regard to the prevention of disease, what process specifically is this bacteria effecting? [PEEK]
- Microbial antagonism (choose best answer) [PEEK]
- is a parasitic interaction between two organisms
- describes generally the harm done by a microorganism to a host
- can be an example of mutualism
- can be an example of parasitism
- all of the above
- none of the above
- A given host-normal flora relationship is considered to be an example of commensalism. What does this suggest to you about the give and take of this relationship?[PEEK]
- A colonic microorganism which bores into its host and by doing so does it damage. This organism has no additional redeeming characteristics. You would describe the symbiotic relationship it has with its host as an example of (chose the best answer): [PEEK]
- an effector of microbial antagonism
- parasitism all of the above
- none of the above
- Name three ways in which a microorganism, in the course of expressing microbial antagonism, might inhibit the growth of another microorganism. [PEEK]
- In the jargon of symbiotic relationships (i.e., words which describe symbiotic relationships in terms of the costs and benefits to the host), what are normal flora? [PEEK]
- Name a specific example of a symbiotic relationship, explicitely name both members. [PEEK]
- Which is not a member of the normal flora of a plant or an animal? (assume all are obligate colonizers of the plant or animal host organism) [PEEK]
- a bacterium
- a virus
- a fungus
- a protozoa
- all are equally likely to be members of normal flora
- Name a part of the body that you would not expect to have an associated normal flora. [PEEK]
- Define commensalism? [PEEK]
- The first bacterium to colonize the large intestine of most newborn's is? [PEEK]
- The acquisition of the normal flora found in the large intestine occurs in stages with, for example, the first microorganisms typically being Lactobacilli acquired from the mother's vagina. Name a species of microorganism (or a specific, crucial characteristic of that microorganism) that must be present in order for the majority of normal adult flora to successfully replicate in this environment. [PEEK]
- The acquisition of the normal flora found in the large intestine occurs in stages. Name a genera or species of microorganism (or a specific, crucial characteristic of one of these microorganisms) that is typically present prior to the acquisition of the majority of normal adult flora. [PEEK]
- Name a genera which is a member of the normal flora of the vagina. [PEEK]
- ii, mutualism. Sure, it sounds like it might be a parasite but the problems this microorganism causes (assuming it doesn't kill the host in the process) are more than offset but its supplying that key organic growth factor.
- microbial antagonism
- iii, can be an example of mutualism
- The host neither gains nor loses in the course of this symbiosis, but the microbe gains a place to live and/or a supply of food. That is, one member of the relationship gains something and the other neither gains nor loses.
- iv, parasitism. Clearly this microorganism is a parasite, but only clearly because I told you that it doesn't do its host any good.
- (i) successfully out competing other microorganisms for limiting nutrients, (ii) successfully out competing other microorganisms for limited space, (iii) actively killing other microorganisms through the production of antimicrobials, (iv) altering the environment in ways which inhibit the growth of other microbes (e.g., acid secreting Lactobacillus in the vagina).
- Mutuals or commensals. That is, they are organisms which spend most or all of their lives in intimate contact with a second organism (the host) which they either they do not hurt or even benefit.
- the simplest answer would be a pathogenic microorganism and you, e.g., humans and Treponema pallidum
- ii, a virus
- Someplace inside the body (e.g., your liver) or someplace moderately external but still with not associated normal flora such as your kidney, your stomach, or the your lungs. That is, the mere presence of microorganisms in any of these locations would be considered to constitute a state of disease.
- Commensalisms are symbiotic relationships between two organisms where one organisms gains and the other organism neither gains nor loses in the interaction.
- Lactobacillus spp.
- Facultative anaerobes such as Escherichia coli.
- Lactobacilli or Facultative anaerobes such as Escherichia coli.
- Lactobacillus is an obvious answer, but other genera include Corynebacterium, Streptococcus, </Staphylococcus, Bacteroides, Clostridium, Candida, and Trichomonas. Note that many of these are opportunistic pathogens, though typically they do not cause disease unless some kind of disturbance occurs, and hence are legitimately included among the normal flora of the vagina.
- Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 392-394, 395-397.
- Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 366-369.