Principles of Modern Microbiology

PRINCIPLES OF MODERN MICROBIOLOGY

Mark Wheelis

ANSWERS TO STUDYQUESTIONS

Chapter 14

Procaryotic Microbes

2. If we want to try to infer something about the common ancestor of all life, there are two approaches that can give us hints. One is the presumption that characteristics shared by all or most descendents are most likely the result of common descent, rather than convergence. Thus we can infer that most of the basic protein synthesizing machinery, the use of DNA for information storage and RNA for gene expression, the specifics of the genetic code, the use of ATP, NADH, and other cofactors in their modern roles, the enzymes of central metabolism and of biosynthesis, and many other common traits were already present in the most recent common ancestor.

We can also, with much less certainty, reason that features of early branching lineages, and of lineages that appear to have undergone much less evolutionary change than others, are likely to be more similar to the common ancestor than more recently diverging and more rapidly changing lineages. This reasoning suggests that the common ancestor was likely a thermophile or extreme thermophile (reasonable on environmental grounds also, as the early earth was much hotter than now). It was probably an anaerobe (also consistent with what we know of the early atmosphere), and it probably used sulfur compounds as electron donor or acceptor. Of course, these conclusions must be very tentative.

4. Many proteobacteria cause human disease: the enteric bacteria, the vibrio group, the pseudomonads, and the rickettsias. So do a number of gram positive bacteria, spirochetes, and chlamydia. No archaea are known to cause human disease. (Note: a number of other groups contain human pathogens; only those that are mentioned in this chapter are included here.)

6. All bacteria contain murein in their cell envelope, with the following exceptions: the planctomycetes (which have a protein envelope), the chlamydia (which have a peptidoglycan that lacks muramic acid), and the mycoplasmas within the gram positive bacteria (which have no envelope, but which have abundant glycolipids).