Microbiology Made Ridiculously Simple--Chapter 3

Front 1

Bacteria have how many chromosomes?

Back 1

ONE

The bacterial chromosome is a dsDNA molecule that is closed in a giant loop. There is only one copy per cell, and thus bacteria are haploid.

There is no nuclear membrane around this DNA.

Front 2

4 Mechanisms by which bacteria exchange genetic fragments are:

Back 2

1. Transformation (Naked DNA fragments from one bacterium, released during lysis, bind to the cell wall of another bacterium. If the recipient bacteria is compotent--(aka able to take up this DNA)--the DNA can incorporate into the recipient geneome assuming sufficient sequence homology. (Bacteria must be closely related). A famous experiment took heat-killed, encapsulated, and virulent strep pneumo and cultured with live, non-virulent (nonencapsulated)pneumococci, and transferred virulence to the non-encapsulated form via this mech.

2. Transduction- occurs when a bacteriophage virus carries bacterial DNA from one bacterium to another. Phages penetrate a host bacteria, and that bacteria's DNA is repressed and eventually destroyed. If for some reason pieces of that bacteria's DNA stay in-tact, it's possible it can be packed into a phage's capsid head, and carried "accidentally" to another material--where that bacterial DNA can incorporate into this secondary host bacterial cell's DNA in a region with sufficient sequence homology.

3. Conjugation (bacterial sex. DNA is transferrede directly by cell-to-cell contact, resulting in an extremely efficient exchange of genetic information. The exchange can occur between unrelated bacteria and is the major mechanism for transfer of antibiotic resistance. For conjugation to occur, one bacterium must have a self-transmissible plasmis--or F plasmid)--which is a circular dsDNA molecule that lies outside the chromosome and carries many genes, including those for drug resistance. F plasmids encode proteins necessary to carry out process for conjugation. Bacteria that carry F plasmids are F+. An F+ donor will pass its F plasmid to an F- recipient making the recipient F+.

4. Transposon insertions- Transposons are mobile genetic elements. You can visualize them as DNA pieces with legs. These pieces of DNA can insert themselves into a donor chromosome without having DNA homology. They can carry genes for antibiotic resistance and virulence factors. Transposons insert into DNA of just about anythin, and they do not replicate indpendently but are copied during their host's DNA transcription. This is important in that a transposon gene that confers a particular drug resistance an move to the plasmids of different bacterial genera, resulting in the rapid spread of resistant strains.

Front 3

Virulent Phage

Back 3

Infect the bacteria, reproduce, and then lead to lysing and killing

Front 4

Temperate Phage

Back 4

Undergoes adsorption and penetration like the virulent phage but then, rather than undergoing transcription, its DNA becomes incorporated into the bacterial chromosome. The DNA then waits for a command to activate.

Front 5

Prophage

Back 5

The integrated temperate phage genome is called a prophage. Bacteria that have a prophage integrated into their chromosome are called "lysogenic" because at some time the repressed prophage can become activated which initiates new phage production, beginning a process that will end with bacterial cell lysis. (So temperate phages are like genetic time bombs).

Front 6

Lysogenic immunity

Back 6

The ability of an integrated bacteriophage (prophage) to block a subsequence infection by a similar phage.

The first temperate phage to infect a bacteria produces a repressor protein. This ensures the first temperate phage is the bacteria's sole occupant.

Front 7

Specialized Transduction and lysogenic conversion

Back 7

Occurs with phage lambda in E.coli.

The lambda prophage inserts its genetic material between E.coli gene for biotin synthesis and galactose synthesis. If a splicing error occurs in either of these genes, then the gene for biotin synthesis may be transferred to another bacteria that does not have that capability.

You will frequently here of this referred to as lysogenic conversion.

Corynebacterium diptheria receives its exotoxin from lysogenic conversion.

Front 8

F' plasmids

Back 8

Occasionally the F plasmid becomes integrated with the cDNA of a neighboring bacterial cell. This can result in one of two things:

1. The f plasmid joins the cDNA and undergoes conjugation with F- cell and transfers the entire bacterial chromosome to the recipient.

or 2. the integrated F plasmid in the neighboring cell may be excised along many points @ a different site from that of integration which can result in an F plasmid that contains just a segment of chromosomal DNA. These are known as F' plasmids. (this is analogous to specialized transduction, because chromossomal DNA is picked up "accidentally" and transferred to other bacterial cells.

Front 9

Medical importance of plasmids

Back 9

Certain plasmids encode enzymes that degrade antibiotics (penicillinase) or generate virulence factors (such as fimbriae and exotoxins)