Microbiology

Microbiology Study Guide

Heterotrophs are organisms that must obtain their carbon in an organic form. Organic carbon originates from the bodies of other organisms . When you obtain carbon from a protein or carbohydrate, you are getting other nutrients along with it. Not all forms of organic carbon are available to all species (most things cannot digest cellulose). Heterotrophs are dependent on other life forms. Common organic molecules that satisfy heterotroph requirements are carbohydrates, proteins, lipids, and nucleic acids. Some organic nutrients available to heterotrophs already exist in a form that is simple enough for absorption (monosaccharides and amino acids). Many larger molecules must be digested by the cell before absorption. Not all heterotrophs can use the same carbon sources; some are restricted to a few substrates, whereas others, like certain pseudomonas bacteria, are so versatile that they can metabolize more than 100 different substrates. Autotrophs are organisms that use CO2 as its carbon source. They are not dependent on other livings things and are considered self-sustaining. These organisms form the base of the food chain that all other life depends on. Even though they obtain carbon in a different manner, they use it in basically the same way as heterotrophs. Examples of autotrophs are plants and photosynthetic microbes. Chemoautotrophs are organisms that require neither sunlight nor organic nutrients. Requirements for Growth Microbial growth is measured in the number of living organisms (size of population), not in the increase in size (weight) of an organism. There are physical, chemical, and gaseous requirements for optimal microbial growth. Physical Requirements • Temperature: A minimum and maximum growth temperature range exists for each species. The optimum growth temperature is the temperature where growth occurs best. o Microorganisms are classified in three groups according to their temperature preferences:  Psychrophilic organisms (psychrophiles) prefer cold temperatures of about 0°C to 20°C  Mesophilic organisms (mesophiles) prefer temperatures at 20°C to 40°C  Thermophilic organisms (thermophiles) prefer temperatures higher than 40°C • pH: pH refers to the extent of acidity or alkalinity of the physical environment. The normal pH range in human body is between 7.35-7.45. o Neutrophilic bacteria have optimum pH between 6.5 and 7.5 and grow well in the body. o Ccidophilic bacteria, such as those in sauerkraut and yogurt, prefer acidic pH < 6.0. Molds and yeasts are among other common acidophilic microorganisms. • Osmotic pressure: Osmotic pressure measures the percentage of salt concentration of the external environment. Normal Saline (N.S.) is 0.9%. Normally, the salt concentration of microbial cytoplasm is about 1%. When the external environment also has a 1% salt concentration, then the osmotic pressure is optimum. Should the external salt concentration rise, as when food is salted, water will flow out of the microbial cytoplasm by osmosis through

©2018

Achieve

Page 21

of 132