The Tenericutes contain a single class named Mollicutes. This class of bacteria lacks a cell wall and are some of the smallest organisms known. Tenericutes are also known as Mycoplasmas and are phylogenetically related to the Firmicutes.  Mycoplasmas are resistant to osmotic lysis which is in part due to sterols. Sterols make the cytoplasmic membranes of Mycoplasma more stable than any other bacterial cytoplasmic membrane.  Some Mycoplasmas contain lipoglycan. Lypogycans are embedded in the cytoplasmic membrane and function in helping stabilize the membrane. They have also been associated with aiding mycoplasmas attachment to the surface of animal cells.

The growth of Mollicutes is not hindered by antibiotics that inhibit cell wall synthesis, as it has no cell wall, but they are hindered by antibiotics that target other cell structures. Most mycoplasmas growth factors include amino acids, purines, vitamin and carbohydrates as carbon and energy sources. The genus Spiroplasma are composed of helical shaped Mollicutes. Though they lack flagella and a cell wall they are able to move by means of rotary motion or slow undulation.

Firmicutes

• mostly composed of gram-positive bacteria
• know as low-GC gram-positive bacteria
• include the lactobacillales that produce lactic acid
• include both nonsporulating and sporulating bacteria- Bacillales and Clostridiales
• Bacillales microorganisms are obligate or facultative organisms
• Clostridiales microorganisms are anaerobic
• cells are either round or rod-like

The proteobacteria are the most metabolically diverse and largest phylum of bacteria. They constitute the majority of known bacteria of medical, industrial, and agricultural significance. All proteobacteria are gram negative and they have a wide diversity of energy generating mechanisms. The proteobacteria can be split into six classes, based on 16S rRNA gene sequences: Alpha-, Beta-, Gamma-, Delta-, Epsilon-, and Zetaproteobacteria. Only Zetaproteobacteria is composed of only one species. All the others contain many genera. Genetic traits are shared in many genomic classes of proteobacteria suggesting that horizontal gene transfer played a major role in shaping the diversity of proteobacteria.

Alphaproteobacteria:

• nearly 1,000 described species
• second largest class of proteobacteria
• most species are obligate aerobes or facultative aerobes
• many are oligotrophic
• the vast majority of species fall into the Rickettsiales,  Rhodospiralles, Sphingomonadales, Caulobacterales, Rhodobacterales, and Rhizobiales 

Betaproteobacteria

• Nearly 500 described species
• third largest class of proteobacteria
• contain species of Burkholderia, Hydrogenophilales, Methylophilates, Neisseriales, Nitrosomonadales, and Rhodocyclales

Gammaproteobacteria

• include a number of important pathogens like Salmonella and Yersinia

Deltaproteobacteria and Epsilonproteobacteria

• contain fewer species and less functional diversity than Alpha-, Beta-, and Gammaproteobacteria
• deltaproteobacteria are primarily sulfate and sulfur reducing bacteria
• Epsilonproteobacteria contain many species that oxidize the H2S  produced by the sulfate and sulfur reducers

Zetaproteobacteria

• composed of only one species- Mariprofundus ferrooxydans
• is an iron oxidizer

Microbial mats are considered to be extremely thick biofilms. Chemolithotrophic and phototrophic bacteria build them. Microbial mats form layers that are composed of different microbial guilds whose activity is dependent upon the amount of light available and other resources like nutrients. Different organisms are living in the mat, and the order in which they are layered depends on nutrient transport controlled by diffusion. These various microorganisms form the layers that make up the microbial mat.

Microbial mats are declining in abundance due to competition of aquatic plants and metazoan grazers. Today, microbial mats develop only in marine environments, where competition is restricted. These environments are hypersaline or geothermal habitats. Because microbial mats grow only in extreme conditions and remote locations, it is difficult to study them today. Microbial mats are sensitive to changing light intensity. Light intensity significantly influences the chemical and biological structure of a microbial mat.

Cyanobacteria is the most abundant and versatile mat builders. They can grow in hot temperatures or cold temperatures. The cyanobacterial mats contain large numbers of primary producers. Primary producers use light to synthesize new organic material from CO2. Cyanobacteria, unlike microbial mats, grow in places that are easily assessable to scientists like Yellowstone or other thermal regions of the world.

Chemolithotrophic microorganisms also build mats. These mats are composed of filamentous sulfur-oxidizing bacteria, which grow on marine sediment. The chemolithotrophic mats oxidize H2S to support energy conservation and autotrophic reactions. The microbial mats on earth composed of Thioploca species on sediments of Chilean and Peruvian continental shelf are thought to be the most extensive microbial mats on Earth.

Microbial mats start as biofilms but as biofilms form on a surface they become more complex and diverse. The diversity reaches its maximum in a microbial mat.

Biofilms are assemblages of bacteria attached to a surface and enclosed in an adhesive matrix. The matrix is made of polysaccharides, proteins, and nucleic acids. These molecules prevent the detachment of cells from surfaces.  Typically biofilms contain many types of bacteria, but can sometimes include only one or two. The biofilm in the mouth, covering the teeth is known to contain as many as 7oo phylotypes of bot archaea and bacteria.

Biofilms form through random collision of cells with a surface. Attachment happens through interactions with the surface and other cells. Before bacteria form a biofilm, they are known, individually, as planktonic cells. Once these cells attach to a surface to form a biofilm,  the bacteria will loose is flagella and become nonmotile.  Cyclic di- guanosine monophosphate is responsible for triggering the transition for plankton into a  biofilm.

Bacteria form biofilm for many reasons. Biofilms increase survival, as it is a means of microbial self-defense, they are resistant to forces that try to remove them from their environment or disconnect them from a surface. Biofilms are resistant to phagocytosis and the immune system of cells. The biofilm allows cells to stay in a certain niche. Another reason biofilms form is because of the close proximity of bacteria cells. Cell to cell communication assists in the survival of the bacteria and the biofilm. Lastly, bacteria from biofilms because it is the default mode of growth in nature. Biofilms are full of nutrients so of course, they are a more typical growth medium for bacteria. You will find individual planktonic cells only if there is a low availability of nutrients in the environment.

Biofilms can be harmful to humans and human lifestyle. They are resistant to the immune system in the body and not all antibiotics, and other microbial agents can penetrate the biofilm. Biofilms have been known to cause cystic fibrosis and are implicated in dental and medical conditions. Biofilms cover medical implants like catheters or artificial joints, causing infection. Biofilms also cause issues in the industrial industry, as they form in pipes and pipelines. The biofilm can cause corrosion of the pipes and cause water in the pipes to become carriers for pathogenic bacteria, like cholera if colonized by the pathogenic agent. Chlorine is used to kill the unwanted bacteria, but it is not always effective.

Flagella are structures that function in cell motility in a liquid medium. In bacteria flagella are long and thin, attached to the cell at one end. Flagella attach to cells at different locations. Various types of flagella connect in different sites of a cell. For example, polar flagella connect at one or both ends of a cell. Another type of flagella, peritrichous flagella, flagella insert at many locations around the cell. Flagellation type is used to characterize bacteria.

The flagella of bacteria are so thin that they cannot be seen with light microscopy unless stained but the electron microscope can see it. During a type of polar flagellation, a group of flagella may rise at one end of a cell. This groups, called a tuft, can be seen by dark field or phase contrast microscopy.

Flagella are not straight but helical and anchored to the cytoplasmic membrane and cell wall. In bacteria, the flagella are made of flagellin, a protein. Flagellin plays a part in shape and wavelength of the flagellum.  The highly conserved amino acid sequence in flagellin suggests that motility evolved early in bacteria.

The flagellum is a rotary motor. In gram-negative cells, the rotor is made up of a central rod that passes through the L ring, P ring, and the MS and C rings. These rings and the central rod comprises the basal body.  These rings are anchored to the LPS layer, peptidoglycan layer, cytoplasmic membrane and the cytoplasm respectively. The second component of a rotary motor, the stator, consisted of Mot proteins that surround the basal body and generate torque.

The energy required to rotate the flagellum come from the proton motive force. The movement of protons across the cytoplasmic membrane and through the Mot complex driven the rotation of the flagellum. About 1000 protons are translocated per a rotating basis.

Gram-positive bacteria have a cell wall that is made primarily of peptidoglycan. In the peptidoglycan cell wall, there are acidic molecules called teichoic acids. The teichoic acids are partially responsible for the negative charge of the gram-positive bacteria cell and the transport of Ca2+ and Mg2+. Some teichoic acids are covelentley bound to membrane lipids and they are called lipoteichoic acids. Due to the majority peptidoglycan cell wall, gram positive bacteria retain the violet dye of the test.

Bacteria cells have a wall that is primarily responsible for its strength. This wall is made of peptidoglycan. Long chains of peptidoglycan construct a sheet around the cell. These chains, made with covalent bonds, provide rigidity in only one direction and only after cross-linking is the cell will rigid in X and Y directions. Peptidoglycan is only found in Bacteria.

Lysosomes destroy peptidoglycan. Peptidoglycan is composed of N-acetylglucosamine and N-acetylmuramic acid. The bonds between the two sugar derivatives are held together by beta-1,4 glycosidic bonds, which are cleaved by lysosomes.

There are two types of bacteria cells gram positive and gram negative. The former has a cell wall made of 90% peptidoglycan. Many gram-positive bacteria have cells walls composed of several sheets of peptidoglycan. The latter has a cell wall with little peptidoglycan. The amount of peptidoglycan affects the results of a test that differentiates between the two bacteria types. The peptidoglycan in the gram-positive cells are dehydrated by alcohol causing the cell wall pores to close, and the violet colored stain to remain.

Continuous Culture

• Open system
• Growth vessel of known fresh medium is added at constant rate while equal volume of spent culture medium is removed at same rate
• Chemostat
  • Device in which growth rate and cell density are controlled independently
    • Dilution rate- rate at which fresh medium is pumped in and spent medium is removed
    • Concentration of a limiting nutrient(Carbon or nitrogen) are present in sterile medium entering chemostat vessel
    • Growth rate controlled by dilution rate
    • Cell yield(density) controlled by concentration of limiting nutrient
  • At high dilution rates
    • Organisms cannot grow fast enough to compete with its dilution and is washed out of chemostat
  • At low dilution rates
    • Cells may die from starvation because the limiting nutrient is not being added fast enough to support minimal cell metabolism
  • Cell population can be maintained in exponential phase for long periods of time
  • Chemostats can used for enrichment and isolation of bacteria from nature

Batch Culture

• Organism in housed in a enclosed vessel
  • Cannot grow exponentially
• Nutrient concentration affects both growth rate and growth yield
  • Low concentrations- Growth rate is submaximal since nutrients cannot be transport into the cell fast enough to satisfy metabolic demand
  • High concentrations- maximal growth rate may be obtained, but the cell density can continue to increase in proportion to the concentration of nutrients in the medium
• Growth Curve describes the growth cycle
  • Lag Phase
  • Exponential phase
  • Stationary Phase
  • Death Phase
• Lag Phase
  • Brief or extended time frame
  • Inoculum taken from old culture
    • Lag is present due to cells being deprived of key nutrients
  • Lag present if inoculum is low of viability(few live cells)
  • Lag is present if cells are damaged but not killed by stressor(Temperature, radiation, toxic chemicals)
  • Lag is observed when microbial culture is transferred from a rich to poor culture
• Exponential Phase
  • Cell population doubles at regular intervals for a brief or extended period
    • Depends on available resources and other factors
  • Cells are in their healthiest state
  • Rate of exponential growth is influenced by environmental conditions( temperature, composition of culture medium) and genetic characteristics
  • Prokaryotes grow faster than eukaryotes
  • Small eukaryotes grow faster than larger ones
• Stationary Phase
  • No net increase or decrease in cell number and thus the growth rate of the population is zero.
  • Energy metabolism and biosynthetic processes may continue at reduced rate
• Death Phase
  • Rate of cell death is slower than rate of exponential growth and viable cells remain in culture for months or years
  • Cryptic Growth- Exponential and Death phases balance each other out