Viruses and Bacteria

What are viruses? Viruses are non-living particles that are composed of a protein coat called a capsid and a nuclei acid core, either DNA or RNA. Viruses are all parasites and must live inside a host cell to replicate themselves.

Viral Structure

Viruses are specific for their host cells. Before a virus can enter it’s host cell it must recognize and attach to receptors on the plasma membrane of the host cell. Viruses have specifically shaped attachment proteins that will match receptors on only specific cells. Example: Hepatits viruses attach to liver cells.

Shapes of Viruses s

Plant viruses enter through injury to plant or insect bites.

Animal viruses enter host cells by endocytosis.

Bacteriophage inject DNA into host cell.

Viral Replication – Lytic Cycle Virus uses the host cell’s energy, raw materials and enzymes to make new viruses. A typical lytic cycle takes about 30 minutes to produce about 200 new viruses. The lytic cycle destroys the host cell.

Steps of the Lytic Cycle

Viral Replication – Lysogenic Cycle In the lysogenic cyle, the viral nucleic acid becomes part of the host DNA; it is integrated into the host chromosome While it is integrated, the viral DNA is called a provirus. It does not affect the host cell, but is copied every time the host cell reproduces. Eventually, the provirus emerges from the host DNA and becomes active.

The Lysogenic Cycle

Examples Lytic viruses: common cold influenzae measles Lysogenic viruses: Hepatitis HIV Herpes

Retroviruses Retroviruses are lysogenic viruses that have RNA as their genetic material. In order to integrate into host DNA, must be able to copy their RNA into DNA - reverse transcription. Retroviruses carry an enzyme called “reverse transcriptase” that enables them to convert RNA into DNA.

HIV infects white blood cells. HIV is a retrovirus that infects a group of disease fighting white blood cells called Helper – Tcells. When HIV emerges from it’s provirus stage, it destroys cells of the immune system and makes the infected person susceptible to many infections and diseases. This is when the person develops AIDS.

Retrovirus lysogenic cycle

Viruses that integrate into regions of a chromosome that controls cell division many cause cancer. Examples: HPV is associated with cervical cancer.

Prions Prions are infectious particles that are composed of misfolded proteins. They are infectious . Cause “mad cow disease” in cows and “Creuzfeldt-Jakob disease” in humans.

Viroids Viroids are small circular RNA particles and found only in plants. They have been found to cause infectious disease in plants.

What came first? The virus or the host? Since viruses cannot replicate without a host cell, viruses probably originated from their host cells.

18.2 Bacteria Prokaryotes are unicellular organisms that have no nucleus or membrane bound organelles. Prokaryotes are divided into two kingdoms: Archaebacteria – these are probably the descendents of the first bacteria that became the ancestors or eukaryotic cells. Eubacteria - the modern bacteria

Archaebacteria – the extremophiles Archaebacteria live in extreme conditions in places that have no free oxygen. Thermophiles - live in high temperature, may be found in boiling mud pots. (Yellowstone National Park)

Halophiles - salt-loving bacteria can exist in very high salt concentrations. These are found in the Great Salt Lake in Utah and the Dead Sea in the Middle East.

Methanogens live in oxygen free environments and produce methane gas. They are found in marshes, lake sediments, under the arctic ice and in the digestive tracts of some grazing animals.

Eubacteria - the modern bacteria Types of Eubacteria based on how they get food. Heterotrophic: these are most bacteria; they are everywhere and use organic molecules as food. Photosynthetic autotrophs: these bacteria make their own food using light (sun) as their energy source. The cyanobacteria are the most common. Chemosynthetic autotrophs: these bacteria breakdown and release the energy stored in inorganic molecules to produce their own food.

Structure of a Bacteria Capsule Gelatin coating around some bacteria that protect them from immune system. Pilus Hair-like structure on the surface of some bacteria that allow them to stick to each other – can exchange genetic information through pili Flagella Whiplike structure that some bacteria use to move Plasmid Small circular piece of DNA in addition to the chromosome that carries a few genes, often antibiotic resistance

Identifying Bacteria Bacteria have three shapes: Cocci which are spherical shaped Bacilli which are rod shaped Spirilli which are spiral shaped

Arrangements of cells help identify bacteria Diplo - means in pairs Strepto - means in chains Staphylo - means in clusters

The Gram Stain is a tool to identify bacteria Bacteria may stain gram positive (purple) or gram negative (pink) based on their cell walls. A gram positive bacteria has one layer of a sugar protein complex in its cell wall which reacts with the purple stain and holds it. A gram negative bacteria has a lipid type layer over the inner sugar protein cell wall that prevents the purple stain from reacting. It is counterstained with a pink stain so that they can be visualized.

The Gram Stain Put bacteria on slide and heat fix the bacteria. Flood slide with crystal violet for 1 minute. Wash with water. Flood slide with iodine for 1 minute. Decolorize for 20 seconds. Flood slide with safranin (pink) for 30 seconds. Pat dry and observe under microscope.

E. coli is a gram negative bacilli Staphylococcus (Staph) is a gram positive cocci

Reproduction of Bacteria Bacteria reproduce asexually by a process called: Binary Fission The bacterial cell grows larger Copies the chromosome Divides the cytoplasm with each cell getting a copy of the chromosome Two identical cells are produced

Conjugation is sexual reproduction in bacteria During conjugation, one bacteria transfers all or part of its DNA to another bacteria. This occurs when the bacteria form a bridge connecting them call a sex pilus. Conjugation results in new combinations of genes and provides variations. Antibiotic resistance is transferred through conjugation.

Bacterial Metabolism Bacteria break down food for energy either using oxygen (respiration) or without oxygen (fermentation) Bacteria that can only live in oxygen are called obligate aerobes. Bacteria that die in the presence of oxygen are called obligate anaerobes. Bacteria that can live in either condition are called facultative anaerobes.

Endospores An endospore is a tiny structure that contains a bacteria’s DNA and a small amount of cytoplasm surrounded by a tough outer covering. They are produced by some bacteria when environmental conditions are unfavorable. They can remain in this condition until favorable conditions return. This is not reproduction because it is the same bacteria.

Importance of Bacteria Nitogen Fixation: Bacteria are the only organisms that can use atmospheric nitrogen and change it into a useable form for plants and animals. All living organisms require nitrogen to make proteins and nucleic acids. Recycling of Nutrient: Bacteria and fungi are the main decomposers returning nutrients back to the environment.

Bacteria cause disease Foods and Medicines Bacteria are used in the production of cheeses, yogurt, pickles, and saurkraut. Some bacteria are used to produce antibiotics. Bacteria cause disease Some bacteria produce toxins (poisons) that cause disease. Examples: cholera, botulism Other bacterial diseases Strep throat - Streptococcus pyogenes Tuberculosis - Mycobacterium tuberculosis Pneumonia - Streptococcus pneumoniae Gonorrhoea - Neisseria gonorrhoeae Tetanus - Clostridium tetani