SCIENCE & TECHNOLOGY

Computer-Aided Image Processing For Bacterial Cell Enumeration

Computer-Aided Image Processing For Bacterial Cell Enumeration

Keywords

Computer-aided image processing, for bacterial cell enumeration, Microbial enumeration

Introduction

A direct count, or direct microscopic count, is one of several methods used in the process called microbial enumeration. Enumeration often simply called counting—may be any method that enables a microbiologist to determine the number of microorganisms in a known weight or volume of material (Koch, 1981, p.114). Two major categories of enumeration available to microbiologists are direct counting and indirect counting.

Discussion

Microbiologists have also developed specific filtration methods for two different laboratory methods: enumeration and motility testing. In enumeration, a microbiologist determines the number of microbial cells present in a very large volume of water. To do this, the microbiologist filters a liter or more of water through a sterile filter. The filter captures all the cells as the water passes through. The microbiologist, then, puts the filter bacteria-side-up on agar or on a pad soaked with liquid medium. After incubating the filter and the agar, the microbiologist counts the number of colonies (called colony-forming units, or CFUs) on the filter's surface. This technique works because nutrients can slowly pass from the agar through the filter's pores and become available for the bacteria growing on the filter's surface. Motility testing is the determination whether a microorganism can move under its own power. A microbiologist simply puts a drop of culture onto a filter that has been placed on top of an agar plate. Motile bacteria will swim through any pores larger than the cell's diameter to reach the nutrients in the agar (Koch, 1981, p.115). The combination of motility and specific pore size has been employed for identifying certain bacteria. For example, treponemes are spirochete (spiral-shaped) pathogens that pass through pore sizes of 0.15 micrometer (µm) that stop almost every other type of bacteria. A microbiologist adds a drop of culture suspected of containing treponemes onto a 0.15-µm-pore-size filter placed over agar. During incubation, the treponemes migrate through the filter into the agar, and this growth appears as a cloudy layer just below the filter. Antibiotics that kill most bacteria but not treponemes can be included in the agar medium to eliminate all species except the treponemes. A similar method has been used for isolating Campylobacter fetus from all other species by using 0.65-µm-pore-size filters overlaid on antibiotic-containing agar.

Filtration also makes use of an apparatus called a Hemming filter, which filters a suspension as it is being centrifuged. The Hemming filter contains two chambers clamped together mouth-to-mouth with a filter in between. One chamber holds the suspension, and the entire apparatus can be fitted into a centrifuge. During centrifugation, gravitational force pushes the suspension through the filter so that the filtrate moves into the second chamber. Hemming filters speed the process of recovering small particles from very concentrated suspensions.

On the other hand, in a direct count, a microbiologist puts a known volume of a microbial suspension into a defined area of a specialized type of microscope ...