Understanding Bacteria - AP Biology

The difference between gram-positive and gram-negative bacteria is the thickness of the peptidoglycan protein layer. Gram-positive bacteria have a thick peptidoglycan layer that retains the gram stain well, making it purple under a microscope. Gram-negative bacteria have a thinner peptidoglycan layer, and do not retain the stain as well as gram positive bacteria. This makes it pink when viewed under a microscope.

The two most common bacterial arrangements are Staph- and Strep-. Staph- is a non-symmetrical arrangement like a cluster of grapes. Strep- indicates a chain of bacteria.

The three most common bacterial shapes are listed below.

Coccus—round or oval

Bacillus—rectangular or rod-shaped

Spirilla (spirochete)—spiral

By breaking down names into arrangement and shape, students can predict or identify the appearance of an organism by its name. In the example Staph indicates arrangement in a chain. Coccus indicates round or oval in shape. Therefore, Staphylococci are round or oval-shaped organisms arranged in a cluster.

The genito-urinary, respiratory, and digestive systems, as well as skin and mucous membranes, are natural physical barriers against infection.

Opportunistic microorganisms are normal fauna that are usually harmless; however, given the opportunity of a breach of any of our natural barriers, these organisms can invade vulnerable systems and cause infection, thus becoming pathogenic.

A pathogen that is an acidophile prefers highly acidic environments and therefore could survive the stomach's 2.0 pH - a lethal barrier to most organisms. The effectiveness of other natural barriers, however, would not be different because the organism is an acidophile.

Aldehydes are powerful, toxic chemicals which work by denaturing DNA and RNA (nucleic acids).

Modern antibiotics are less toxic and work by disrupting cells' production of ribosomes. Common household cleaners, containing chlorine, work by oxidizing cell membranes. Older antibiotics, such as penicillin, work by dissolving cell walls. These modes of destruction are also known as mechanisms of cell death.

Leukocytes can be broken down into the following categories and cells types.

Spores can survive most methods of control and are resistant to cold and heat. Pasteurization is a heating process that destroys some pathogenic microorganisms in foods and beverages, but does not kill spores. Boiling is not considered a reliable lethal control because the destruction of bacterial spores cannot be guaranteed. Cold temperature is a non-lethal means of microbial control that can slow multiplication or induce stasis, but cold is only effective while conditions are maintained.

Incineration is the use of extreme temperature to reduce matter to ash and is an effective means of sterilization.

Detergents, or soaps, are non-lethal microbial controls. They emulsify lipids and oils, making it easier to wash them away. The action of washing away substances is strictly a mechanical mode of action. Detergents provide only static control, and have no killing power.

We need to know the different between hypertonic, hypotonic, and isotonic. An environment is said to be hypertonic is there are more solute outside of the cell than inside. Since these solutes do not cross the cellular membrane very easily, but water can cross, water will travel out of the cell to balance the concentrations. This would cause the cell to shrink and shrivel.

The opposite is true for a hypotonic scenario, in which the cell will swell and maybe burst as water flows from the environment into the solute-filled cell. An isotonic environment is one in which solute concentrations are even, so there is no net movement of water into or out of the cell.

Prokaryotes create their own proteins, which means that ribosomes are present in the cell. Viruses, however, rely on the ribosomes of the host cell in order to create proteins. This means that ribosomes are not present in viruses, but are present in prokaryotes.

DNA and RNA will both be found in prokaryotes. Some viruses will contain DNA, while others will contain RNA. Attachment proteins allow viruses to bind to cell membranes, but also can allow one cell to bind to another.

Recall that bacterial ribosomes contain a 30S and a 50S subunit. The ribosome itself is a 70S ribosome, as the combination of the two subunits results in a smaller mass than the two separate units. Eukaryotic ribosomes contain a 40S and 60S subunit and form an 80S ribosome.

Flagella are used for mobility by both prokaryotes (bacteria) and eukaryotes. In humans, you can find these structures in sperm cells, which are propelled by the motion of flagella. Similarly, bacterial cells move through the environment with the aid of flagella. The key difference between a bacterial and eukaryotic flagellum is its composition. Bacterial flagella are made up of a protein called flagellin, whereas eukaryotic flagella are made up of microtubules, formed from the protein tubulin.

Formation of a bridge between two cells occurs during conjugation. The bridge is called a sex pilus, which is used to carry genetic material from one bacterial cell to the other.

Transduction is another mode of reproduction, but involves transmission of genetic material through a vector. Vectors are generally host cells or virions that carry parts of the bacterial genome known as plasmids.

The third type of bacterial reproduction is called transformation. In transformation a bacterial cell engulfs and incorporates genetic material from its surroundings.

Translation is not a type of bacterial reproduction; it is the process by which proteins are made from mRNA.

Similar to eukaryotes, bacterial cells are in accordance with the central dogma of molecular biology. The flow of genetic information involves replication, transcription, and translation. Bacterial cells have to replicate their genetic material (replication), undergo transcription to convert DNA to mRNA, and go through translation to produce proteins from the mRNA.

Bacterial cells do not undergo mitosis or meiosis. Cell division can occur through other modes, such as binary fission. Since bacteria lack nuclei and other complex structures, their processes for cellular division are much more abbreviated and do not follow the same steps as eukaryotic mitosis.

A plasmid replicates itself by being introduced to a bacterium and inserting its genetic information. The bacterium then undergoes its replication process with the genetic information that was injected by the plasmid and in this way the plasmid is cloned. Bacteria do not undergo mitosis, rather, they undergo a process called binary fission to produce identical daughter cells.

Recall that pink stained cells are gram-negative, whereas purple stained cells are gram-positive. Based on color, we can conclude that cell A is gram-negative (pink) and cell B is gram-positive (purple). One of the characteristics of gram-positive cells is that they have a thick cell wall. The thick peptidoglycan cell wall stops the gram stain from leaking out, resulting in a darker color.

Since cell B is purple, we can conclude that it is gram-positive and has a thick cell wall.

First, remember that all bacterial cell walls contain peptidoglycan. The difference between gram-positive and gram-negative bacterial cell walls rises from the composition of peptidoglycan. Gram-negative bacteria appear pink when observed through a microscope, not purple. Also, gram-negative bacteria have thinner cell walls than their counterparts.

Gram-positive bacteria have half as many cell membranes as gram-negative bacteria. This is true because gram-negative bacteria contain an extra cell membrane outside their cell wall. Gram-negative bacteria have two cell membranes, whereas gram-positive bacteria only have one.

If you place a cell in a hypotonic solution it means that the solution contains less solute (and more water) than the cell’s cytosol. Recall that osmosis causes water to go from a region of high water content (few solutes) to a region of low water content (more solutes).

In this case, the water will go from the solution to the inside of the cell, causing the cell to swell.

The name staphylococcus aureus contains the term "coccus," which indicates that the bacteria is round-shaped.

Rod shaped bacteria are classified as bacilli and helical bacteria are classified as spirilla.

Only gram-negative bacteria have an outer membrane. This outer membrane contains lipopolysaccharides. Gram-positive bacteria have only one membrane.

Teichoic acids are present only in gram-positive bacteria. They contribute to the thickness of the peptidoglycan layer.

Both types of bacteria have peptidoglycan. Gram-positive bacteria have a thick layer, due to the presence of teichoic acids. This allows them to retain the gram stain and appear purple. Gram-negative bacteria have a thin peptidoglycan layer, causing them to appear pink under gram staining.