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130 SEcTioN ii Microbiology ` microbiology—basic bacteriology Microbiology ` microbiology—basic bacteriology

Bacterial genetics
Transformation Competent bacteria can bind and import short Degraded
pieces of environmental naked bacterial uncombined Recipient DNA
DNA
chromosomal DNA (from bacterial cell Donor DNA
lysis). The transfer and expression of newly
transferred genes is called transformation. Naked DNA Recipient cell Transformed cell
A feature of many bacteria, especially
S pneumoniae, H influenzae type b, and
Neisseria (SHiN).
Adding deoxyribonuclease degrades naked
DNA, preventing transformation.
Conjugation
F × F – F plasmid contains genes required for sex pilus Single strand
+
+
and conjugation. Bacteria without this plasmid Sex pilus transferred
+
–
are termed F . Sex pilus on F bacterium Plasmid
−
contacts F bacterium. A single strand
of plasmid DNA is transferred across the F cell F cell F cell F cell F cell F cell
–
+
+
+
+
–
conjugal bridge (“mating bridge”). No transfer
of chromosomal DNA.
+
Hfr × F – F plasmid can become incorporated into Plasmid incorporates Transfer and replication
into bacterial DNA of part of the chromosome
bacterial chromosomal DNA, termed high-
frequency recombination (Hfr) cell. Transfer Plasmid
of leading part of plasmid and a few flanking F + cell F – cell Hfr cellF – cell Hfr cell F – cell Hfr cell Recombinant
chromosomal genes. High-frequency F – cell
recombination may integrate some of those
–
bacterial genes. Recipient cell remains F but
now may have new bacterial genes.
Transduction
Generalized A packaging “error.” Lytic phage infects Cleavage of Bacterial DNA packaged
Lytic
bacterium, leading to cleavage of bacterial phage Bacteria bacterial DNA in phage capsids
DNA. Parts of bacterial chromosomal DNA
may become packaged in phage capsid. Phage
infects another bacterium, transferring these
genes.

Release of new phage Infects other Genes transferred
from lysed cell bacteria to new bacteria
Specialized An “excision” event. Lysogenic phage infects Viral DNA Viral DNA
incorporates in
Phage particles
bacterium; viral DNA incorporates into Lysogenic Bacteria bacterial DNA carry bacterial DNA
phage
bacterial chromosome. When phage DNA
is excised, flanking bacterial genes may be
excised with it. DNA is packaged into phage
capsid and can infect another bacterium.
Genes for the following 5 bacterial toxins are
encoded in a lysogenic phage (ABCD’S): Group
A strep erythrogenic toxin, Botulinum toxin,
Cholera toxin, Diphtheria toxin, Shiga toxin. Release of new phage Infects other Genes di erent from
donor and recipient
from lysed cell
bacteria

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