Chapter 13. Chapter 13: Evolution and Diversity Among the Microbes

Amoebas are tiny cells. Their small size allows the exchange of gases, including oxygen and carbon dioxide, across their cell membrane at a high enough rate to meet their metabolic needs, so they do not require a respiratory system. Larger, multicellular organisms do require a respiratory system because, without it, gas exchange cannot occur at a rate fast enough to meet their metabolic needs.

Microbes are genetically very diverse. They live in just about every type of environment imaginable, including at extreme temperatures and pressures, and they live on just about any type of food available. Thus, it is difficult to make any generalizations about microbes. There are huge numbers of microbes living on earth, with a huge number of species, many of which are still to be identified.

Bacteria have a cell envelope made up of a plasma membrane and, in most cases, a cell wall, enabling them to control what moves into and out of the cell. These structures also allow bacteria to live in harsh environments and still maintain internal conditions within a range that allows them to survive.

Conjugation: A copied piece of DNA is transferred between bacteria, even if they are not of the same species. Transduction: A virus inadvertently transfers bacterial DNA from one bacterium to another. This occurs as a virus picks up DNA from one host and, as it infects another host, transfers the DNA to that cell. Transformation: As bacteria die they release their DNA into the environment, and it can be scavenged by other bacteria. This new DNA may code for traits the cell does not already have.

Aerobic bacteria require oxygen to carry out cellular respiration, whereas anaerobic bacteria do not. Bacteria that can live aerobically when oxygen is available and anaerobically when oxygen is not available are called facultative anaerobes.

The characteristic taste and texture of yogurt result from the use of bacteria in yogurt production. The process uses bacteria that metabolize milk sugar, called lactose, for energy and produce lactic acid as a by-product. Lactic acid has a low pH and causes milk proteins to denature, resulting in the characteristic taste and texture.

The source of this cholera outbreak was studied in 1854 by Dr. John Snow, who identified the source as the Broad Street pump, which was delivering contaminated water. The handle of the pump was removed so no one else could use it as a source of water. It was Dr. Snow who persuaded the authorities to remove the handle, and as a result the number of new cholera cases fell dramatically.

Some strains of Staphylococcus aureus have become resistant to most of the antibiotics available today. Untreatable staph infections have spread from hospitals into the larger community and are killing more people than HIV infections.

(a) Many people are unaware that they are infected with an STD because symptoms may be mild or absent, and they may inadvertently pass the disease to another person. (b) It is nearly impossible to completely eliminate all STDs because both partners need to be treated simultaneously, and this is hard to arrange if people are unaware of their infection.

Any two of the following answers are possible. Bacteria and archaea have chemical and/or structural differences in: (a) their DNA, (b) their plasma membranes, (c) their cell walls, and (d) their flagella.

“Extremophile” refers to the ability of these organisms to tolerate extremes in their physical and chemical environments.

Because many archaea live in extreme environments, it can be challenging to replicate these conditions in the laboratory so as to successfully grow and study these organisms.

The protists were the first organisms to develop a nucleus, a structure with a double membrane, called the nuclear envelope, that encloses the cell’s chromosomes. It is thought that the double membrane could have formed from the fusion of infoldings of the cell’s plasma membrane.

The ciliates, including Paramecium, are a group of protists considered to be animal-like, since they move and hunt other microorganisms, much like animals.

Plasmodium goes through a series of developmental stages, each of which has different surface proteins. These proteins are what an immune system uses to identify foreign organisms. It takes time for an immune system to recognize a foreign organism by its surface proteins, so the constant change in surface proteins with each stage of Plasmodium development means the immune system has to repeatedly identify and react to new proteins.

A virus is composed of a piece of DNA or RNA surrounded by a protein coat. Unlike living organisms, a virus is not made up of cells, cannot replicate on its own, carries out no metabolic activities within its protein coat, lacks the structures found in cells, and cannot detect and respond to stimuli in its environment.

The genetic material making up a DNA virus is relatively stable over time, since there are enzymes present to correct replication mistakes. There are no comparable enzymes that correct replication errors for RNA, therefore RNA viruses have a higher rate of mutation, making them more difficult for the immune system to fight.

The types of glycoprotein present on the surface of the virus determine which organism(s) it can infect and which tissue(s) of a multicellular organism it can infect.