Microbiology, Lec 19.

Announcements:

Pass back tests and discuss some of the questions.......

mean, 68

low, 29

high, 96

In order to bring the mean of this test up to that of the last test, I am adding 3 points to everyoneÕs score. Two of those points = that I will now give everyone credit for question 9 (discuss why in class...), although this exact same question may appear on the final and then only answer a will be accepted.

If I had to give out grades right now (based on the mean of the first two exams) the curve would be approximately as follows:

85 - 100 = A

73 - 84 = B

63 - 72 = C

53 - 62 = D

The rest of the semester will be devoted to the interactions between microbes and Eucarya, with special emphasis on diseases of humans.

Symbiotic Associations

We will start by discussing the different types of symbitic associations and then move on to the "normal microbiota" of animals and plants and then lead into to disease causing organisms. We have already alluded to the fact that all animals (and plants) are covered and filled with microbes (remember elephant example). Most of the microbes that inhabit animals and plants are not pathogenic and are involved in mutualistic or commensalistic relationships with their hosts.

Some definitions:

Mutualism=> Symbiotic relationship in which both partners gain (e.g. legume-rhizobium relationship, lec. 13); mycorrhizae (see below); nitrifying bacteria (syntrophism, see below, lec. 13); lichens; ruminants; termites (lec. 13) etc. etc. ....

Lichens (Fig. 28.2)

Review the cow (Fig. 28.5 & 28.6 a,b)

Tube worm - chemoautotroph mutualism (Fig. 28.4, lec. 13)

Loose mutualistic relationships in microbiology are sometimes called syntrophism; the example of nitrifying bacteria could be characterized as a syntrophic relationship......

Show example of Syntrophobacter and Methanospirillum. The syntrophic condition may be very common in nature and may be why most natural bacteria cannot be cultured easily in the lab.

Commensalism=> Symbiotic relationship in which one partner gains and the other is mostly unaffected (e.g. many gut bacteria are commensals with us).

Parasitism is the condition where one organism (the parasite) is living at the expense of another (the host) and usually is harming the host. A parasite that causes detectable damage to the host is called a pathogen. An infection is caused by a pathogen growing in a host. One pathogen that we have already talked about was Agrobacterium tumefaciens in lec. 15.

Before we go off on too many pathogenic relationships let's look at least one more mutualistic one - e.g. the cow (pgs. 570 - 572)....

Figures 28.5 and 28.6 show how a cow works (see also lec. 13). Go over the various compartments of the cow's digestive system. Cows can get nutrition from things like cellulose (high C:N) because they have a rumen that is filled with anaerobic microorganisms. Like other anaerobic environments there are many fermentative microbes in the Rumen. These microbes produce fermentative endproducts like propionic acid, lactic acid etc. (see Figure 28.6) that are then absorbed by the cow. Of course there are also methanogenic bacteria in the rumen and therefore cows can belch about 100 liters of methane per day!

A Note on Knotobiotic Animals

(read pgs. 571 - 573)

Gnotobiotic (Greek for "known biota") animals are also sometimes call "germ-free" animals.....used in medical and nutritional research
They are obtained via cesarean section (since the fetus is sterile) and then raised in microbe-free "bubbles".

Some observations that relate to the function of the normal microbiota of animals...

1) knotobiotic animals have enlarged cecums
2) need high amounts of vitamin K and some B vitamins
3) very susceptible to pathogens (very low inoculum needed to cause infection)

The normal human microbiota.

(pg. 573)

Let's start by looking at Figure 28.8. The first thing to realize is that parts of the human body that are connected with the outside world are inhabited by microorganisms. Thus, the respiratory tract, the digestive system, the genitourinary system and of course the skin are all colonized by microbes. There are at least 100 trillion bacteria associated with a normal adult, a number greater than the number of animal cells present! Prior to birth, the fetus is free of microorganisms. Most resident microbes are acquired from the skin of adult humans.

Skin. We discussed some of the inhabitants of the skin in lec. 7. Most of the normal residents of the skin are gram + Bacteria (e.g. Corynebacterium, Propionibacterium, Staphylococcus and Micrococcus spp.), with Staphylococcus being the dominant genus on many people (see overhead). The normal bacterial residents of the skin help to prevent skin infections by lowering the pH of the skin, excreting chemicals toxic to other organisms and by using up nutrients that pathogens could otherwise use.

Respiratory Tract. The inhabitants of the nostrils are often quite similar to those of facial skin, with staphylococci dominating. Deeper inside the nose, there are more sreptococci and even high numbers of some gram - bacteria, such as avirulent strains of Neisseria meningiditis and Hemophilus spp.

The mouth is, of course, a seething metropolis of microbes. Constant saliva production and swallowing tend to flush bacteria that don't stick to surfaces. There are about 100 million bacteria per ml of saliva. Streptococcus mutans is important in plaque production on teeth and S. salivarius (see Yogurt) stays in the mouth by colonizing the tongue. We'll come back to plaque and S. mutans a little later in the course.

The lower respiratory tract (trachea, bronchi and lungs) is kept mostly free of microbes by the "muco-ciliary escalator".

Gastrointestinal Tract. The extreme acidity (pH = 2) and digestive enzymes kill most microbes that pass through the stomach. Even so there are resident microbes of even the stomach (less than 1000 per ml. in healthy humans; e.g. Lactobacilli and some acid acidophilic yeast) including some pathogens (e.g. Helicobacter pylori in people with ulcers).

As food moves through the different sections of the 5 meter-long small intestine bacterial populations go up. Many of the bacteria in the small intestines are facultative anaerobes (e.g. E. coli) and therefore most of the oxygen is removed from food by the time it gets to the large intestine.

The large intestine is basically an anaerobic chamber (sort of like the sludge digestor at a sewage treatment plant) populated by countless microbes. Most of the microbes there are strict anaerobes. The dominant gram negative bacteria are in the genus Bacteroides (lec. 6) and the dominant gram + organisms include our old friends Review Bifidobacterium bifidus, Lactobacillusspp. and Clostridiumfrom lec. 7. There are also large populations of methanogens (Archaea, lec. 8) in most human colons (about 30% of adult humans have high populations of methanogens, and thus are more gaseous than the other 70% of the population).

We benefit greatly from the inhabitants of the gastrointestinal tract. In addition to aiding in digestion, these critters also give us vitamins such as K, B12, biotin and riboflavin. They also exclude pathogenic organisms by filling all available niches. About 30% of the mass of feces consists of microbial cells. Adults excrete trillions of microbes per day!

Review also Probiotics by reading Box 28.2 and brief article from "Science News". See also the Yogurt lecture.

Genitourinary Tract. Bacteria are normal inhabitants of the lower part of the urethra in both males and females. The vagina of adult females is colonized by acid producing lactobacilli (see lec. 7 and Yogurt) and other acid tolerant bacteria and yeast. The normal microbiota of the vagina maintain a pH of about 4.5 which is low enough to inhibit many pathogens.

In contrast to areas that are connected to the outside world, areas that are protected from the external environment are not normally inhabited by microbes. In a healthy human there should not be significant numbers of microbes in organs, blood, internal tissues or the lymphatic system. Of course the blood and lymph systems are constantly clearing invading microbes from your body so transient microbes are found there.

A few more helpful definitions:

Bacteremia = Presence of viable bacteria in the blood (usually transient)

Septicemia = growth or persistence of bacteria (and/or bacterial toxins) in the blood

Infection = Growth of a pathogenic organism in a host.