MID 1Author: Frank Lowy Bacterial Classification, Structure and Function Introduction The purpose of this lecture is to introduce you to terminology used in microbiology. The lecture will: 1. Cover different classification schemes for grouping bacteria, especially the use of the Gram stain 2. Describe the different types of bacteria 3. Discuss bacterial structure and the function of the different bacterial components 4. Discuss the distinguishing characteristics of Gram positive and Gram negative bacteria. For this lecture you should focus on the major concepts and not on the names of the different bacteria. They are mentioned as illustrations of different principles. You will see them all again as the course progresses. Classification Systems The classification of bacteria serves a variety of different functions. Because of this variety, bacteria may be grouped using many different typing schemes. The critical feature for all these classification systems is an organism identified by one individual (scientist, clinician, epidemiologist), is recognized as the same organism by another individual. At present the typing schemes used by clinicians and clinical microbiologists rely on phenotypic typing schemes. These schemes utilize the bacterial morphology and staining properties of the organism, as well as O2growth requirements of the species combined with a variety of biochemical tests. For clinicians, the environmental reservoir of the organism, the vectors and means of transmission of the pathogen are also of great importance. The classification schemes most commonly used by clinicians and clinical microbiologists are discussed below. Scientists interested in the evolution of microorganisms are more interested in taxonomic techniques that allow for the comparison of highly conserved genes among different species. As a result of these comparisons a phylogenetic tree can be developed that displays the degree of relatedness of different organisms. A relatively new application of this technology has been the recognition and characterization of noncultivatable pathogens and the diseases that they cause. Phenotypic classification systems: There is a chart at the end of these lecture notes on the general phenotypic classification of many of the clinically important bacteria. This is provided as a reference. By the end of the course you will be able to recognize most of these microorganisms. Gram stain and bacterial morphology: Of all the different classification systems, the Gram stain has withstood the test of time. Discovered by H.C. Gram in 1884 it remains an important and useful technique to this day. It allows a large proportion of clinically important bacteria to be classified as either Gram positive or negative based on their morphology and differential staining properties. Slides are sequentially stained with crystal violet, iodine, then destained with alcohol and counter-stained with safranin. Gram positive bacteria stain blue-purple and Gram negative bacteria stain red. The difference between the two groups is believed to be due to a much larger peptidoglycan (cell wall) in Gram positives. As a result the iodine and crystal violet precipitate in the MID 1thickened cell wall and are not eluted by alcohol in contrast with the Gram negatives where the crystal violet is readily eluted from the bacteria. As a result bacteria can be distinguished based on their morphology and staining properties. Some bacteria such as mycobacteria (the cause of tuberculosis) are not reliably stained due to the large lipid content of the peptidoglycan. Alternative staining techniques (Kinyoun or acid fast stain) are therefore used that take advantage of the resistance to destaining after lengthier initial staining. Growth Requirements: Microorganisms can be grouped on the basis of their need for oxygen to grow. Facultatively anaerobic bacteria can grow in high oxygen or low oxygen content and are among the more versatile bacteria. In contrast, strictly anaerobic bacteria grow only in conditions where there is minimal or no oxygen present in the environment. Bacteria such as bacteroides found in the large bowel are examples of anaerobes. Strict aerobes only grow in the presence of significant quantities of oxygen. Pseudomonas aeruginosa, an opportunistic pathogen, is an example of a strict aerobe. Microaerophilic bacteria grow under conditions of reduced oxygen and sometimes also require increased levels of carbon dioxide. Neisseria species (e.g., the cause of gonorrhea) are examples of micraerophilic bacteria. Biochemical reactions: Clinical microbiology laboratories typically will identify a pathogen in a clinical sample, purify the microorganism by plating a single colony of the microorganism on a separate plate, and then perform a series of biochemical studies that will identify the bacterial species. Serologic systems: Selected antisera can be used to classify different bacterial species. This may be bas