Microbiology,

Lec. 18

Announcements:

I have to cancel my office hours today, but I will have extra office hours today from 3-4 and Wed. from 12-1 and by appointment.

Some practice questions can now be found at: Practice Exam 2

Today we will:

1) tie up a few things from last time
2) discuss some issues related to Environ. Micro. (including some review of physiological concepts), and
3) spend some time reviewing for the EXAM THIS THUR.

Environmental Microbiology

In the last lecture we talked about the fate and effects of wastes such as sewage. Today we will discuss what happens to wastes that are harder for humans and microbes to deal with. We will refer to these compounds as being xenobiotic (= foreign to life) and/or recalcitrant molecules (read pgs. 900 - 902). Xenobiotic compounds are often toxic to life and are also often hard for microorganisms to metabolize (because they contain molecular arrangements that not normally encountered in nature). Thus, many of these compounds can accumulate in the environment and continue to be a hazard for many years. Most pesticides that are produced in the U.S. today are required to be biodegradable so that they do not accumulate in the environment.

biomagnification (see pg.841)

Rings and halogenation make molecules more recalcitrant.

Lindane and Mirex,

2,4-D (3 months to degrade)

2,4,5-T (3 years) Fig. 42.21

Polychlorinated Biphenyls (PCBs), Fig. 44.21

1) Non-specific and "sloppy" enzymes.

We will use the example of the monooxygenase of methanotrophic bacteria (all are Proteobacteria that specialize on methane oxidation, see pgs. 473-475). Illustrate the degradation of trichloroethylene (TCE) via methanotrophic bacteria. TCE is a very common pollutant (Table 33.2) from degreasing and dry cleaning operations.

Another example of non-specific enzymatic attack are ligninases produced by certain Basidiomycetes like Phanerochaete chrysosporium (see Box 44.2, p. 955). This enzyme probably evolved to attack the random (and "natural") structure of lignin (see Fig. and Table 40.40), but it has also been shown to attack many xenobiotic chemicals as well (although quite slowly).

One thing to remember about both of these examples is that the microbe carrying out the biodegradation is not necessarily gaining any carbon or energy from their actions. Thus, some of these types of reactions may be purely fortuitous. They may also serve as detoxification reactions (just as similar enzymes in our livers hydroxylate toxic chemicals). In the case of P. chrysosporium it is thought that they are just trying to get lignin out of the way so that they can access nitrogen compounds in the plant tissues. The only evidence for this is that P. chrysosporium only produces ligninases when they are N limited.

2) Use of xenobiotic chemicals for C and e- using modifications of natural metabolic pathways.

Naphthalene mineralization (see catabolic plasmid example from lec. 15) by Pseudomonas putida

Note that a dioxygenase is involved and that once the ring is broken it is a simple matter to produce compounds like pyruvate and acetyl-CoA. Naphthalene is a very toxic human-made chemical, but it also occurs in oil deposits. Thus microbes have had millions of years to figure out how to deal with compounds like benzene.

There are compounds that microbes can mineralize for growth that probably haven't been around for millions of years......e.g.

Pentachlorophenol (compare to phenol on overhead) is broken down by enzymes that probably evolved from those used by bacteria to breakdown naturally occurring phenolic compounds.

Under anaerobic conditions some chlorinated compounds may be acting as e- acceptors during the process of reductive dechlorination. This type of anaerobic respiration may be important in the breakdown of PCBs.

Bioremediation as an industry.

Many of the versatile critters that we discussed today are presently being used to treat soils and waters at the more than 50,000 hazardous waste sites in the U.S. In some cases bioreactors are being used to "pump and treat" contaminated ground waters. Contaminated soils are often treated by adding selected bacteria and nutrients to the soil to increase the natural rate of breakdown of xenobiotic chemicals. In some instances just adding nutrients (e.g. oxygen to anaerobic ground water or sediments, Fig. 44.21) can stimulate the biodegradation of pollutants. All of the above are expensive and bioremediation of our past environmental mistakes is now a multi-billion dollar (tax payer funded) industry in this country.