Life and Diversity: Bacteria

Transcription

1 Life and Diversity: Bacteria Dr. Waggoner BIOL 1400 We discussed archaea (top) and bacteria (bottom), which are both prokaryotes: cells without nuclei. Most bacteria, like most archaea, are at most a few microns (millionths of a meter) in size. Even under the best light microscopes, most bacteria look like dots, and can t be seen very well without staining. Exceptions to this rule are the cyanobacteria, a.k.a. blue-green pond scum, whose cells are much larger. Note that there s not much detail visible inside; these are still prokaryotes... 1

2 Electron microscopes, however, have revealed much of what we know about bacterial structure. This cross-section through a bacterium shows that there s not an awful lot of complex parts inside it.... Surrounding the cell is a flexible cell membrane, and usually a stiffer cell wall just outside the membrane. Cell wall composition is different in different types of bacteria, but usually includes at least some of a protein-sugar compound called peptidoglycan. A few bacteria, called rickettsias, have no cell wall. (Rickettsias are of interest in part because they include organisms that cause typhus, Rocky Mountain spotted fever, and a few other diseases.) Outside of the cell wall there may be a layer of slime, the capsule (which can be important in determining whether the bacterium causes infection or not). These capsules are visible under the light microscope. 2

3 Reproduction is typically by fission (splitting in two). This is a cross-section through a bacterium caught in the act of fission and here s the happy couple afterwards. Note that a form of sexual reproduction can also take place. Two clumps of DNA Furrow where cells are pinching in two Some bacteria are able to swim, with long, threadlike, corkscrew-shaped flagella. Finally, some bacteria can form resistant endospores when life gets rough (seen here as bright areas within the cells). 3

4 Endospores are extremely tough and thick-walled capsules that can withstand almost any conditions. When conditions improve, the endospore comes back to life. (Since endospores can resist heat, some bacteria can survive the canning process and grow in canned foods, potentially causing food poisoning.) Classifying bacteria can be a complex business, but it s often useful to use the shape of the cells as one criterion. These are cocci, or dot-shaped bacteria... These are bacilli, or rodshaped bacteria... And these are spirilla (spiralshaped bacteria) and a vibrio (a comma-shaped or bean-shaped bacterium). 4

5 Very long spiralshaped bacteria are known as spirochaetes. The bacterium that causes Lyme disease (that disease you can catch from tick bites) is a spirochaete. So is the bacterium that causes syphilis. Bacteria can further be identified by how they group together during growth. These are cocci growing in bunches, or staphylococci these are cocci growing in pairs, or diplococci and these are cocci growing in chains, or streptococci. 5

6 WHO CARES? Bacterial shape, size, growth form, and cell wall structure are all used to identify pathogens (disease-causing agents) in human patients, animals, plants, food, water, sewage, and so on. How bacteria make you sick A few bacterial diseases: Syphilis, gonorrhea, stomach ulcers, cholera, botulism, bubonic plague, Lyme disease, legionnaire's disease, tuberculosis, strep throat, diphtheria, impetigo, leprosy... Often, it's not so much the bacteria themselves that cause illness, but toxins that the bacteria produce. Exotoxins are released by bacterial cells (examples: tetanus, botulism, staph, diphtheria) Endotoxins are certain components of bacterial cell walls (example: salmonella) The 19th-century German bacteriologist Robert Koch came up with rigorous experimental procedures for determining whether a bacterium causes a particular disease. Koch s Postulates The same bacteria must be present in every human or animal with a particular disease. The bacterium must be grown in the laboratory in a pure culture -- one with no other cells present. The disease must appear in experimental animals when the cultured bacteria are injected into them. The same bacteria must be isolated from the experimental animals after they get the disease. Robert Koch ( ) 6

7 Koch used these postulates to pinpoint the bacteria that cause anthrax (his drawings of anthrax bacteria are shown at left), cholera, tuberculosis, and others which led to many treatment and prevention methods, and which won him a Nobel Prize in Important Disclaimer Bacteria don t only cause diseases in fact, the overwhelming majority of bacteria are completely harmless Bacteria are everywhere water, air, soils, plant and animal bodies... Many bacteria require oxygen to live; these are called aerobes. Other bacteria do not need oxygen, or may even be poisoned by oxygen. These are anaerobes. Many bacteria are saprotrophic they break down dead organisms, causing decay and releasing nutrients back into the environment. Beneficial bacteria Escherischia coli bacteria in your intestines help you digest; they also make vitamin K and vitamin B 12. Bacteria living inside the roots of plants, such as alfalfa, take up nitrogen gas from the air and convert it into a form the plant can use (nitrates) A few bacteria produce antibiotic drugs, such as streptomycin and nocardicin. Bacteria used in the food industry convert milk to buttermilk and yogurt, and wine to vinegar. They also put the holes (and the taste) in Swiss cheese. The lumps on these soybean roots are formed by bacteria that live inside the plant's roots which take nitrogen gas and convert it into a form that the plant can use. This is common in legumes (beans, peas, clover, alfalfa, etc.) 7

8 Beneficial bacteria Bacteria break down wastes in sewage treatment and in septic tanks. Strains of bacteria exist that can feed on petroleum. These have been used to clean up oil spills. Others can extract metals from mining waste, and are becoming used in mining and in environmental clean-up. As a byproduct of photosynthesis, blue-green bacteria, or cyanobacteria, produce much of the oxygen that we and other organisms breathe. Marine bacteria such as this recently discovered species, Alcanivorax borkumensis, are capable of breaking down crude oil. They exist around natural crude oil seeps on the seafloor, but become extremely abundant in the wake of disastrous oil spills. It ll probably take a while, but bacteria like this will eventually clean up the Gulf of Mexico... 8