IPC - Module 1 Flashcards
What is a microorganism?
A Microorganism is:
An organism too small to be seen without the aid of a microscope.
Relatively simple in structure and often unicellular (single-celled).
Also called a “microbe”, “germ” or “bug”.
Some people in the medical community may use the slang term “bug” when referring to a microorganism. For example “What kind of bug does the patient have?” They are not referring to an insect in this case, but rather an infection causing microorganism.
Groups of Microorganisms
The four groups of microorganisms are:
Protozoa
Fungi
Bacteria
Viruses
Protozoa
Protozoa are unicellular creatures that move either by flagella or amoeboid motion. Each cell has a nucleus and is enclosed by a plasma membrane. Protozoa live in water and soil and feed on bacteria and small particles.
Some protozoa live in our bodies without causing any harm but a few are responsible for disease. Most people are familiar with Giardia infections, commonly called “beaver fever.” Giardia attach to the intestinal wall by two sucking discs resulting in diarrhea because the food cannot be broken down and absorbed properly. Giardia move by flagella. Some amoeba will also invade the intestinal wall and cause diarrhea. Occasionally they move into deeper tissue. These protozoa are usually picked up in warm countries with poor sanitation. The amoeba move by extension of pseudopods and do not have flagella
Fungi
Fungi can be thought of as non-photosynthetic plants. Each cell has a nucleus and is enclosed by a rigid cell wall. The fungi are a very diverse group of microorganisms that can be simply divided into yeasts or molds
Yeasts are unicellular oval structures that reproduce by budding. Many types of yeast are used in the food and beverage industry for making breads and wines. They all look the same under the microscope. Some yeasts are capable of causing disease in humans with Candida being the most common. Candida may cause oral thrush, vaginal discharge, skin infections, pneumonia and even death.
Molds are multicellular structures that form visible clumps of growth. Bread and cheese molds are typical examples. The mold starts out with long tubular structures that eventually produce spores. These spores are visible as grey, blue – green or black growth. Human infections caused by the mold-type of fungi usually involve skin.
Athlete’s foot and ring worm are infections caused by this type of fungus.
Bacteria
Bacteria are tiny unicellular organisms typically surrounded by a rigid cell wall. They do not have an organized nucleus but carry out all of the activities necessary for growth and reproduction.
Bacteria are ubiquitous which means that they are found just about everywhere there is moisture and nutrients. Many bacteria grow in or on humans and are an essential part of our body flora. However, many other bacteria cause infections.
Sore throat or “Strept throat” is caused by a type of bacteria called Streptococcus. Certain types of Staphylococcus cause skin abscesses called boils. Almost everyone has heard of food poisoning caused by the Salmonella from poultry. Figure 1.1.5 shows what bacteria look like under the microscope.
Viruses
Viruses are even smaller than bacteria and have very simple structure. Viruses are unable to grow and reproduce on their own and must rely on a living host cell to replicate the viral parts. Animal, plant and bacterial cells all serve as host cells for viruses.
A number of human diseases are caused by viruses. Influenza and colds are viral infections that inflict discomfort on everyone at some point during their life. More serious viral infections include hepatitis, rabies and HIV.
Treatment
Each group of microorganism, when causing an infection, requires a different drug for treatment. Protozoal infections are treated with antiprotozoal drugs, fungal infections are treated with antifungal drugs; bacterial infections are treated with antibiotics and viral infections are treated with antiviral drugs.
Size of Microorganisms
Most microorganisms are measured in metric units called micrometers.
One micrometer equals 1/1,000 of a millimeter. A portion of a metric ruler is shown in Figure 1.1.6, with the larger units being centimeters and the smaller units being millimeters. All microorganisms are smaller than 0.1 mm which equals 100 micrometers
Size of Protozoa
15-20 Micrometers
Size of Fungi
5-10 Micrometers
Size of Bacteria
0.3 - 5 micrometers
Size of Viruses
0.3 - 5 micrometers
Shapes of Bacteria
Most disease-causing microorganisms found in the hospital setting are bacteria. Bacteria can be placed in three groups based on their shape. Their shape is maintained by the rigid cell wall.
Cocci (pleural), coccus (singular)
Spherical or round cells
Rods or bacilli (bacillus)
Rectangular shaped boxes
Spirilla (spirillum)
Curved or spiral shaped rods
Gram Reaction of Bacteria
Gram Stain
Bacteria can be divided into two groups on the basis of a Gram stain. The stain has been used for over 100 years to stain bacteria and make them more visible when viewed under the microscope. Once the staining procedure is carried out, one group of bacteria appear dark bluish-black in color and are called gram positive. The other bacteria are a pink to red color and are called gram negative.
Importance of Gram Reaction
Looking at the gram reaction and shape of bacteria is often the first step in identifying them in the laboratory. Bacteria can be divided into six groups, based on the gram reaction and the shape of the cells:
gram positive cocci
gram negative cocci
gram positive rods ( bacilli)
gram negative rods (bacilli)
gram positive spirilla
gram negative spirilla
The gram reaction determines the effectiveness of certain antibiotics. Penicillin G, for example, is quite effective in killing gram positive bacteria but is relatively ineffective against gram negative bacteria.
Gram stain results assist the physician in initial selection of an appropriate antibiotic.
The gram reaction determines the effectiveness of certain disinfectants as well. Some disinfectants are more effective against gram positive than gram negative bacteria.
Bacterial Endospores
A small number of bacteria (a few Gram positive rods) are able to produce a special type of spore within the bacterial cell. These spores are typically called endospores to differentiate them from fungal spores borne on the ends of hyphae.
Bacterial cells that do not contain endospores are often referred to as vegetative cells. Even Gram positive rods that are able to form spores exist as vegetative cells when they are actively growing and multiplying.
Sporulation
Sporulation takes place when certain nutrients are depleted. One copy of the genetic material and a tiny amount of cytoplasm is enclosed by a layer of insulating material and the whole structure is covered with several compact layers of spore coat.
Endospores may remain dormant for days, months and even years without nutrients or moisture. Many of the bacteria found in dust, cereals, grains and soil exist as endospores.
A “viable” endospore is able to germinate (grow) into a vegetative cell when moisture and nutrients are provided. One spore will germinate into one vegetative bacterium.
The spores are found ubiquitously (everywhere) in the soil and normally do not cause infections as they require special conditions to vegetate, such as lack of oxygen. Gangrene and tetanus result when spores are introduced deep into injured tissue where blood flow has been disrupted and oxygen levels are low. Unlike gangrene and tetanus, anthrax will grow in the presence of oxygen, but requires other pre-disposing conditions such as damaged tissue or inhalation in to the lungs.
The significance of endospores in sterilization and disinfection:
Endospores are quite resistant to heat. Most vegetative bacteria are killed by exposure to moist heat at 60-80°C for 10 minutes, but spores will survive these high temperatures. Some spores are killed by boiling, but others require a temperature of 121°C for 12-15 minutes for destruction.
Endospores are more resistant to disinfectants than vegetative bacteria. Low level disinfectants may not kill endospores and high-level disinfectants require extended exposure times.
Endospores are also very resistant to destruction by cold (viable spores have been discovered in the intestines of mammoths found in Siberian glaciers).
Endospores are also resistant to ultraviolet light, acids, alkalis and detergents.
Growth of Bacteria - Binary Fission
Almost all bacteria reproduce by a process called binary fission
Generation Time
Generation Time is the time it takes for binary fission to occur. This is the time it takes for the population to double. For example:
One bacterium becomes two bacteria
Forty bacteria become eighty bacteria
Generation time is not the same for all bacteria and is also affected by temperature and available nutrients. Rapidly growing bacteria under ideal conditions have a generation time of 15-30 minutes.
The generation time is related to the rate at which bacteria cause disease. The bacteria responsible for gas gangrene have a very short generation time, believed to be about 8 minutes. Once infection is established in human tissue, a whole limb may be destroyed in a day. The bacteria responsible for tuberculosis, on the other hand, have a long generation time (12-24 hours) and take weeks to produce disease
Generation time & colonies
The generation time also determines the amount of time required for bacteria to form visible growth on culture media. A colony is a visible mass of bacteria that develops on the surface of a solid culture medium after a period of time and represents all the descendants of a single bacterial cell. The culture medium provides necessary nutrients for bacterial growth. Rapidly growing bacteria will form colonies within 18-24 hours.
Bacterial Growth Curve
Bacteria do not go on growing at their maximum generation time forever. There would be no room for anything else on the surface of the earth if they did. If one cell divided every 20 minutes for 24 hours, there would be 1 x 1021 cells with a mass of 4,000 tons. Fortunately, reproduction is curtailed due to lack of nutrients, space and the build up of toxic waste products. The maximum number of bacteria possible is about 1 x 109 per mL or gram.
When bacteria are introduced into a new medium or environment, they go through four specific phases of growth. These phases make up the bacterial growth curve,
4 stages of bacteria growth curve
- Lag Phase
When bacterial cells are placed in a new environment, there is little or no increase in numbers for a short period of time. This time is necessary for the cells to adapt to the new environment. The lag phase is usually a few hours but may be much shorter or much longer in some situations.
The lag period for bacteria in food is considered to be about two hours. This allows foods to be left out on a serving table for that long without much worry of bacterial growth and possible food poisoning.
- Log Phase
This is a period of maximum growth of bacteria. All the cells are dividing at a constant rate. - Stationary Phase
The bacteria eventually exhaust their supply of nutrients and stop growing and multiplying. Even if nutrients are not used up, the end products of bacterial growth may accumulate and prevent further growth. The number of live bacteria stays constant during this period. - Death Phase (also called the Phase of Decline)
Eventually the bacterial cells start to die and the number of live cells decreases. In some cases, most of the bacteria die in a short period of time but in other cases, some live bacteria can linger for weeks, months or even years. The bacteria that can survive the longest are those that form spores.
The Growth Curve Related to Infection
Growth curves are plotted using laboratory cultures but the same sequence of events often takes place when a microorganism invades a host:
There are usually no disease symptoms for a period of time immediately following introduction of the microorganism into the host. This represents the incubation period of the disease, also called the lag phase of the growth curve.
This is followed by the onset of symptoms which may be fever, sore throat, swollen lymph glands, rash, vomiting or diarrhea depending on the infectious microbe involved. This is the acute stage of the disease and corresponds to the log phase. With the assistance of the patients defenses and medical intervention, most patients recover in this phase. However, if these fail, the patient dies during this acute stage.
The stationary phase follows, which is a period of time during which the symptoms are no worse but there is also no sign of recovery.
Finally the symptoms subside and recovery is on the way. This is the death phase (phase of decline) – death of the microorganism, not the host. The convalescent period of the disease covers the time needed for complete recovery. Recovery time will vary with different diseases, and infectious microorganisms often continue to be discharged during this period.
