Bacterial Genetics Flashcards Preview

Microbiology/Immunology > Bacterial Genetics > Flashcards

Flashcards in Bacterial Genetics Deck (19)

Genetic material of bacteria

-chromosomal DNA- a large circle of DNA containing almost the entire genome of the bacterium (typically 2,000-5,000 genes)

-plasmids- circles of DNA containing small number of genes, (typically 3 or 4) that are not essential to the bacterium. They are extrachromosomal

-bacteriophages- viruses that infect bacteria and carry a small number of genes (typically 3-4) that are not essential to the bacterium. They may be extrachromosomal or integrated


Bacterial vs Human Genetics

Nucleus: Bacterial-No; Human-Yes
Number of genes: B- 4,000; H- 25,000
Ploidy: B- Haploid; H-Diploid
Chromosomes: B-One, Circular; H-Many,Linear
Extra-chromosomal: B-plasmids/phage; H-mtDNA


Expression of bacterial genes

-similarly to eukaryotic cells, genes consist of specific DNA sequences which can be transcribed into RNA, which can be translated into protein
- in eukaryotic DNA each gene has its own regulatory elements. In bacteria, genes with related functions all share the same regulatory elements and are known as an operon
-transcriptional regulation. In eukaryotic cells the expression of gene is regulated by proteins that interact with the gene promoter. In bacteria expression is often regulated by metabolic products or deficiencies directly. This can be by positive or negative regulatoion
-post translational regulation


Phage and plasmids can encode virulence factors

-enterotoxin (E. colim V. Cholerae)
-exfoliatin (S. aureus)
-erythrogenic toxin (S. pyogenes)
-neurotoxin (C. tetani)



-unique for each bacterial species, but is flanked by conserved sequences
-universal regions can be used as primer binding sites for PCR or sequencing
-intervening regions can be used to identify bacteria at genus or species level
-this allows microbiome-wide identification of bacteria


Different features of expression of Human and Bacterial genes

Human- individual genes, multiple transcription factors, enhancers, RNA is spliced, post-translational control (proteins modified)

Bacteria- operons, operator/repressors, none, not, none


Bacterial genotypes

Biosynthetic genes- ala means organism synthesizes alanine

Catabolic genes- lac meansit can digest lactose

Drug resistance genes- cat means it resists chloramphenicol


Regulation of gene expression in bacteria

-nutritional status
-cell-surface sensing
-quorum sensing



- the promoter is separated from the genes by an opertator-which regulates expression of the operons
-the expression of the lac operon genes is normally prevented by a repressor protein
-"inducible expression"- when lactose is present the genes to metabolize it are induced by the lactose


Mutations in bacteria

-chemical modifying agents (base-modifying agents, base analogs, intercalating agents)
-physical agents (X rays, UV light)
-often carcinogenic to eukaryotic cells- basis of the Ames test- test it with Salmonella his- , negative some spontaneous revertants, positive result- increased revertants near test chemical

-can lead to resistance to antibiotics- very rare


Genetic exchange of bacteria experiment

-SII- capsule, lethal to mice
-SI- no capsule, mice live
-heat killed SII- mice live
-combined SI and heat killed SII- lethal



-DNA released from dead bacteria may be taken up non-specifcally by live bacteria
-if it is not digested by restriction enzymes it may be incorporated into the recipient genome by homologous recombination
-this is the mechanism that allows the capsule gene of S. pneumonii to be transferred, the DNA might be chromosomal DNA or plasmid DNA



-some bacteria carry a large plasmid- the F factor- which can exist as an extrachromosomal element or can be integrated into the chromosome
-the F factor also encodes sex pili and the genes for the process of conjugation
-during conjugation the F plasmid, plus bacterial DNA from the integration site, is transferred from the donor bacterium (male) through the sex pilus into the recipient bacterium (female)
-bacteria can be F- (female), F+ (male), Hfr (Male with integrated plasmid)
-transfers many antibiotic resistant genes in intestinal bacteria


Prevention of recombination

-resistriction modification system

Bacterial enzymes:
-modify their own DNA by methylation
-digest DNA from other sources (non-methylated)
-repair damaged DNA and splice in DNA that is modified (methylated)



-transmission of DNA between bacteria by bacteriphage
-lytic phage can infect bacteria and can cause lysis- this can release DNA which can be taken up by the bacteriophage and be carried into another bacterium- this is known as generalized transduction
-lysogenic phage can insert themselves into the bacterial chromosome (lysogeny, producing a prophage in the chromosome) and later can excise themselves with some adjacent bacterial DNA which can be carried into another bacterium- this is known as specialized transduction. This mechanism transfers toxins of of diphtheria, botulism, cholera, scarlet fever
-Segments of non-chromosomal DNA that carry genes for antibiotic resistance are called R-factors, larger ones are called F factors



-within bacterial chromosome, some genes are arranged in elements that can move around the genome
-these encode a transposase enzyme gene, a repressor that stops expression of the transposase, and typically a drug resistance gene
-the relocation of the transposon might bring it into a site where the F plasmid or where a bacteriophage might integrate, thus allowing the transposon to be moved to another bacterium


Pathogenicity islands

-some regions of a bacterial chromosome may have several adjacent genes that contribute to pathogenesis of a disease
-many gram negative rods may have pathogenicity islands or may not
-thus some strains of E coli express many toxins and are pathogenic while other strains are not toxic
-they are presumably transferred between bacteria but do not have an intrinsic relocation mechanism

-toxin delivery system


Restriction enzyme

-used for genetic engineering for cutting and splicing DNA for production of new plasmid or viruses
-examples of practical uses include the recombinant hep B vaccine made in yeast, recombinant insulin made in E. coli, and virus vectors for gene therapy


Multidrug resistance of S. aureus

-S. aureus-transposition through F plasmid to Penicillin, Aminoglycosides, Trimethoprim
-E. Fecalis- transposition through F plasmid to Vancomycin A and Vancomycin
-S. aureus- mutation from the methicillin receptor is lost