Microbial Growth

Bacterial Cell Division

• growth is defined as an increase in the number of cells
• binary fission (“binary” to express the fact that two cells have arisen from one)
• the time required for this process is called the generation time ��

Chromosome Replication and Partitioning

• Each circular chromosome has a single site at which replication starts called the origin of replication,
• Replication is completed at the terminus, which is located directly opposite the origin
• group of proteins needed for DNA synthesis assemble to form the replisome at the origin�

Cytokinesis

• Septation is the process of forming a cross wall between two daughter cells
• (1) selection of the site where the septum will be formed;
• (2) assembly of a specialized structure called the Z ring, which divides the cell in two by constriction;
• (3) linkage of the Z ring to the plasma membrane and components of the cell wall;
• (4) assembly of the cell wall-synthesizing machinery; and
• (5) constriction of the Z ring and septum formation.

MreB forms spiral filaments around the inside of the cell that help determine cell shape

FtsZ assembles in the center of the cell to form a Z ring,

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MinCD oscillates from pole to pole,

thereby preventing the formation of an off-center Z ring

Archaea

• They can stain either gram positive or gram negative
• spherical, rod-shaped, spiral, lobed, cuboidal, triangular, plate-shaped, irregularly shaped, or pleomorphic.
• Some are single cells or may be filaments or aggregates
• diameter from 0.1 to over 15 µm,
• Multiplication may be by binary fission, budding, fragmentation, or other mechanisms
• aerobic, facultatively anaerobic, or strictly anaerobic.
• psychrophiles, mesophiles, and hyperthermophiles

• Crenarchaeota
• mostly hyperthermophiles
• optimum is greater than 80°C
• primary producers in their habitat
• Euryarchaeota
• physiologically diverse group
• Methanogens (anaerobes) ; halophilic (obligate aerobes) ;
• hyperthermophiles - Thermococcus and Pyrococcus

Extremely Halophilic Archaea

• Halobacterium
• inhabits environments high in salt.
• Requires 1.5 M (about 9%) or more sodium chloride (NaCl)
• salt evaporation ponds and salt lakes e.g. Great Salt Lake in Utah (USA)
• gram-negative rod shaped, reproduce by binary fission, and do not form resting stages or spores.
• gas vesicles that allow it to float in its salty hypersaline habitat
• obligate aerobes.

Halobacterium…

• large plasmids containing up to 30% of the total cellular DNA
• GC base ratio of these plasmids (near 60% GC) vs chromosomal DNA (66–68% GC).
• use amino acids or organic acids as energy sources and require a number of growth factors (mainly vitamins)
• pump large amounts of K+ from the environment into the cytoplasm.
• high content of the acidic (negatively charged) amino acids aspartate and glutamate in the glycoprotein
• cytoplasmic proteins of Halobacterium are highly acidic, but it is K+

• can catalyze a light-driven synthesis of ATP
• Bacteriorhodopsin- Halobacterium salinarum and some other haloarchaea. insert it into their cytoplasmic membranes.
• Retinal is also attached, give purple hue.
• oxygen-limiting growth conditions

Halobacterium…

Halobacterium…

• Halorhodopsin is a light-driven chloride (Cl-) pump that brings Cl- into the cell as the anion for K+.
• Sensory rhodopsins, control phototaxis
• Proteorhodopsins, functions like bacteriorhodopsin

Methanogenic Archaea

• methanogens, microorganisms that produce methane (CH4) as an integral part of their energy metabolism
• methanogenesis

• cell wall pseudomurein composed of methanochondroitin
• methanogens are obligate anaerobes
• mesophilic and nonhalophilic; but extremophilic has been described�

Thermococcales

• Thermococcus and Pyrococcus,
• Pyrococcus
• Spherical
• 70 and 106°C with an optimum of 100°C
• Obligately anaerobic chemoorganotroph
• Proteins, starch, or maltose are oxidized as electron donors
• S is the terminal electron acceptor and is reduced to hydrogen sulfide (H2S)�

Crenarchaeota

• Sulfolobus
• grows in sulfur-rich acidic thermal areas
• temperatures up to 90°C and at pH values of 1–5
• aerobic chemolithotroph
• oxidizes H2S or S to H2SO4 and fixes CO2 as a carbon source�

Spirochetes

• Spirochaeta, Treponema, Cristispira, Leptospira, Borrelia
• gram-negative, motile, tightly coiled Bacteria, typically slender and flexuous in shape
• widespread in aquatic environments and in animals.
• Some cause diseases, including syphilis,
• motility is conferred by flagella that emerge from each pole - endoflagella �

Chlamydia

• Chlamydia and Chlamydophila are obligate intracellular parasites with poor metabolic capacities;
• gram-negative-type cell walls
• they have both DNA and RNA; ��

Rickettsia

Mycoplasma

• Mycoplasmas are the smallest and simplest self-replicating bacteria.
• The mycoplasma cell contains minimum set of organelles essential for growth and replication: a plasma membrane, ribosomes, and a genome consisting of a double-stranded circular DNA molecule.
• Unlike all other prokaryotes, the mycoplasmas have no cell walls, and they are consequently placed in a separate class Mollicutes (mollis, soft; cutis, skin).

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• The name Mycoplasma is derived from the Greek word mykes (fungus) and plasma (formed)
• Mycos :Fungus like form of branching filaments & Plasma :Denoting plasticicity of their shape.
• An older name for Mycoplasma was Pleuropneumonia-Like Organisms (PPLO),
• causative agent of contagious bovine pleuropneumonia (CBPP).

• In 1898 Nocard and Roux reported the cultivation of the causative agent of CBPP, which was at that time a grave and widespread disease in cattle herds. The disease is caused by M. mycoides subsp. mycoides .

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GENERAL CHARACTERISTICS

• Very small(0.2-0.3 µm) .
• Can pass through bacterial filters.
• Lack a rigid cell wall.
• Bound by a single trilaminar cell membrane that contains a sterol.
• Extremely pleomorphic varying in shape from coccoid to filamentous to other bizzare forms.

• Mycoplasma, Ureaplasma, Spiroplasma and Anaeroplasma cannot synthesize their own cholesterol and require it as a growth factor in culture medium.
• Acholeplasma synthesizes carotenol as a substitute for cholesterol, but will incorporate cholesterol if it is provided.
• Insensitive to cell-wall active antibiotics such as penicillins and cephalosporins.
• Limited biosynthetic capabilities due to a small genome.

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• Multiply by binary fission. However, cytoplasmic division may lag behind genome division. This results in the formation of multinucleate filaments and other shapes.
• Do not possess flagellae or pili
• Non sporing
• Stain poorly with Gram stain. Can be stained with Giemsa and Dienes methods.
• Are considered as stable L forms by some researchers but this hypothesis is still not fully accepted.

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GENOME

• The mycoplasma genome is typically prokaryotic, consisting of a circular, double stranded DNA molecule.
• The Mycoplasma and Ureaplasma genomes are the smallest recorded for any self-reproducing prokaryote.
• In some mycoplasmas the number is estimated at fewer than 500, about one sixth the number of genes in Escherichia coli.
• Mycoplasmas accordingly express a small number of cell proteins and lack many enzymatic activities and metabolic pathways.
• Their nutritional requirements are correspondingly complex, and they are dependent on a parasitic mode of life.

CULTURAL CHARACTERISTICS

• Aerobes and facultative anaerobes except Anaeroplasma which is strictly anaerobic.

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• For primary isolation, an atmosphere of 95% Nitrogen and 5% Carbon dioxide is preferred.

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• They can grow within a temperature range of 22-41°C, the parasitic species growing optimally at 35-37°C.

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• For fermentative organisms, the initial pH of the medium is adjusted to 7.3-7.8, for arginine metabolizing organisms it should be around 7 and for ureaplasmas, range of pH should be 6-6.5.

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• All mycoplasmas cultivated and identified thus far are parasites of humans, animals, plants, or arthropods.

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• The primary habitats of human and animal mycoplasmas are the mucous surfaces of the respiratory and urogenital tracts and the joints in some animals.

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• Although some mycoplasmas belong to the normal flora, many species are pathogens, causing various diseases that tend to run a chronic course.

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MYCOPLASMAS AS NORMAL FLORA