Bacterial Reproduction, Metabolism, and Genetics PDF

Bacterial reproduction, genetics and metabolism How do the bacteria replicate? The bacterial cell is divided by binary fission 1. Replication of the bacterial DNA and separation of the copies 2. Elongation of the cell, movement of the DNA...

Bacterial reproduction, genetics and metabolism How do the bacteria replicate? The bacterial cell is divided by binary fission 1. Replication of the bacterial DNA and separation of the copies 2. Elongation of the cell, movement of the DNA copies toward the ends of the cell. Synthesis of the cytoplasmic membrane and the cell wall. 3. Formation of the dividing septum (the process requires PBPs and autolysins) 4. Division of the two daughter cells; cellular material and external structures are distributed among the daughter cells The generation time varies from 50-60 minutes and follows the equation 2n 16777216 cells  Cell division Incomplete cleavage of the septum can cause the bacteria to remain linked, forming chains (streptococci) or clusters (staphylococci).  BACTERIAL SPORES Some Gram positive - but never Gram negative-bacteria – members of the genera Bacillus and Clostrium are spore formers. The spore is a dormant state of the cell produced by the vegetative form under harsh environmental conditions, as loss of nutritional requirements (depletion of alanine) The spore is: o a dehydrated multi-shelled structure that protects and allows the bacteria to exist in “suspended animation” o extremely resistant to drying, UV radiation, heat (~ 100°C), attack by enzymes and chemical agents o able to remain in "metabolic hibernation" for a long time (centuries): the enzymatic activity is reduced and macromolecular syntheses are absent → antibiotics are ineffective Bacillus Clostridium  BACTERIAL SPORES Endospore is structurally and chemically more complex than the vegetative cell, having the following layers: o Exosporium is the outermost layer made up of proteins that encloses spore coat o Spore coat is a thick double layered covering that encloses cortex; it consists of spore specific proteins (keratin-like protein→resistance to adverse environmental conditions traces of lipids and peptidoglycan o Cortex is made up of loosely arranged peptidoglycan, cytoplasmic membrane residues, and of calcium bound to dipicolinic acid. o Core or spore protoplast is the innermost part of the spore; it consists of core wall, cytoplasmic membrane, dehydrate cytoplasm (gel-like), high amount of calcium bound to dipicolinic acid, high percentage of small acid soluble proteins (SASPs) bound to the nucleoid  Calcium dipicolinate The calcium dipicolinate has the function of reducing the presence of water inside the endospore, facilitating dehydration. In addition, the complex is intercalated in the DNA making it stable at denaturation by heating. Sporulation  Germination The germination process occurs in three stages: 1. Activation: even when placed in a nutritionally rich environment, bacterial spore will not germinate unless first activated by agents that damage (→ permeabilization) the spore coat (heat, abrasion, acidity, and compounds containing free sulfhydryl groups) 2. Initiation: once activated, a spore will initiate germination if the environmental condtions are favorable. Different species have evolved specific receptors recognizing effectors as signiling a rich medium. Binding of the effectors activates an autolysin that degrades the cortex peptidoglycan → water is taken up, calcium dipicolinate is released and spore structures are degraded 3. Outgrowth: degradation of the cortex and outer layers results in the emergence of a new vegetative cell consisting of the spore protoplast with its surrounding wall. A period of active biosynthesis follows wich terminates in cell division. BACTERIAL SPORES  The Bacterial Genome DNA chromosome + extrachromosomal genetic elements DNA CHROMOSOME Circular, double-stranded and supercoiled DNA molecule Haploid genome →mutation of a bacterial gene will have a more obvious effect on the cell. PLASMIDS BACTERIOPHAGES (phages) Circular, double-stranded and Viruses that infect bacteria supercoiled DNA molecule THESE ELEMENTS CAN BE TRANSMITTED FROM ONE CELL TO ANOTHER  The Bacterial Genome Vertical Transmission Genetical elements are distributed to daughter cells following cell division (DNA chromosome →equally; plasmids →independently and randomly) Horizontal Transmission Genetical elements are distributed with a unidirectional mechanism from a donor cell to an acceptor cell: o transformation o conjugation o transduction TRANSFORMATION Genetic transformation is the incorporation of naked DNA from the extracellular environment. o a competent cell binds a dsDNA fragment through the DNA-binding proteins o the exogenous DNA is cleaved by endonucleases to smaller dsDNA fragments and then ssDNA fragments o DNA uptake requires the attivation of autolysin protein o internalization of foreign DNA and alignment with homologous sequences of the genome o homologous recombination  CONJUGATION Conjugation is a one-way transfer from a donor cell to a recipient cell through sex pilus. o F+ cell attaches to F- cell with its sex pilus (conjugative bridge). The pilus draws the cell together o the cells contact one another o one strand of the plasmid DNA transfers to the recipient, during the rolling circle replication o recipient cell synthesizes a complementary strand to become F+. The donor cell synthesizes a complementary strand restoring its complete plasmid  Replication of plasmids DNA rolling circle replication requires: o replication origin (oriC) o nicking enzyme o DNA-dependent DNA polymerase III o DNA-dependent DNA polymerase I A typical DNA rolling circle replication has five steps: 1. Circular dsDNA will be "nicked". 2. The 3’-end is elongated using "unnicked" DNA as leading strand (template); 5’-end is displaced. 3. Displaced DNA is a lagging strand and is made double stranded via a series of Okazaki fragments 4. Replication of both "unnicked" and displaced ssDNA. 5. Displaced DNA circularizes. New DNA is synthesized semiconservatively, and proceeds unidirectionally  CONJUGATION o Hfr+ cell attaches to F- cell with its sex pilus (conjugative bridge). The pilus draws the cell together o the cells contact one another o during the rolling circle replication, a fragment of the bacterial genome is transferred together with a strand of the plasmid o the transferred DNA aligns with homologous sequences of the recipient’s genome o homologous recombination  TRANSDUCTION Transduction is a genetic transfer mediated by bacterial viruses (bacteriophages) that pick up fragments of bacterial DNA and package them into bacteriophage particles BACTERIOPHAGES  TRANSDUCTION Transduction can be classified as: o specialized if the phages transfer particular genes (usually those adjacent to their integration sites in the genome), thus during a lysogenic cycle o generalized if incorporation of DNA sequences is random because of accidental packaging of host DNA into the phage capsid, thus during a lytic cycle HORIZONTAL TRANSFER OF GENETIC MATERIAL IN BACTERIA The horizontal transfer from bacteria to bacteria of resistance genes represents the most frequent mechanism of diffusion of resistance to anti- bacterial drugs  BACTERIAL METABOLISM the vast majorty of bacteria of interest in pharmacy and medicine are chemoheterotrophs: they obtain carbon, nitrogen and energy by breaking down organic compounds  BACTERIAL METABOLISM O2 not required O2 not present Aerobic respiration Anaerobic respiration Aerobic respiration → when O2 is the molecule at the end of the sequence of respiratory reactions that finally accepts the electrons Anaerobic respiration → when other molecules (nitrate, fumarate) finally accept the electrons Fermentation (anaerobic process) → when organic molecules are the final electron acceptors  BACTERIAL FERMENTATION Fermentation of pyruvate by different microorganisms results in different end-products. The clinical laboratory uses these pathways and end-products as a means of distinguishing different bacteria.  BACTERIAL METABOLISM Entner-Doudoroff (ED) Embden-Meyer-Parnas Pentose phosphate pathway (EMP) pathway or (PP) pathway Glycolysis  Aerobic – Anaerobic bacteria Depending on the possibilities of adequately performing the catabolic processes necessary for the production of energy, the bacteria are divided into aerobic bacteria and anaerobic bacteria (with different degrees). Obligate aerobes o must grow in the presence of oxygen o they cannot carry out fermentation Facultative anaerobes o can perform both aerobic respiration and fermentation o they switch from aerobic respiration to fermentation if oxygen supplies are depleted Obligate anaerobes o they perform fermentation o they are killed by oxygen Microaerophilic bacteria/aerotolerant anaerobes o they perform anaerobic respiration o grow in low concentrations of oxygen (2-10%)