Automated Plate Counts

The spread plate technique may be automated by the use of a piece of apparatus known as the spiral plater. An agar plate is rotated on an Archimedean spiral whilst being inoculated. The volume of the sample decreases (and is therefore effectively diluted) as the spiral moves towards the outer edge of the plate. A specialised counting grid relating the area of plate to the sample volume enables colonies in the appropriate sector to be counted. An electronic colony counter travelling in the same Archimedean screw may be used. 

This technique removes the need for multiple sterile dilutions, and can save a considerable amount of sample preparation and time when large numbers of samples are being processed. The apparatus is expensive and can only be justified where a large number of samples need processing. 

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Savings in time and materials can be made by removing the need for the dilution series in the standard plate colony count. Various methods based on the Thompson plate loop method have been described. In the technique, two loops that retain 0.01 and 0.001 ml are dipped into the sample. A Petri dish is positioned under each loop and the loops are flushed with diluent, agar is then added, and the contents of the Petri dish are mixed. These machines can plate 10 and 10 dilutions from 300 samples per hour. The technique is suitable for enumerating bacteria in the range 3000-300 000 ml Manufacturers of one commercially available automated plate loop machine (PetriFoss) claim that the relationship with the reference method has a correlation coefficient of 0.99. 

Injection volumes of sample and flow rate of the mobile phase are parameters which may be varied under conditions of the preliminary study. Injection volumes greater than 40 pi produce a decrease in the current per unit concentration. Consequently, injection volumes in the range of 20-40 pi are employed in both conventional and automated systems described later in this paper. An increase in flow rate leads to an increase in current because of increased convection. With a conventional Ci8 column, and for flow rates above about 2 ml min , the variation of current with flow rate is marginal and the decrease in effective plate count occurring as the flow rate increases means that flow rates in the range of 1-2 ml min are optimum for determining copper. 

An alternative method which can also be automated by the use of the Titertek supernatant harvester (see Appendix 3) involves the measurement of radioactive chromium released into the culture medium from killed cells. The harvester consists of a set of absorbent cylinders aligned so that they may be inserted into the wells of a microtitration plate (Appendix 3). Once the supernatant in the wells has been absorbed the cylinders are transferred to counting vials and the amount of radioactive chromium released from the cell monolayer is estimated. Cells take up 51 Cr sodium chromate rapidly and the excess is readily washed away by rinsing in culture medium. 

Output formats Cuvette or plate reader, microscope images, videos, radioactivity (scintillation or Geiger counting) Plate reader— Usts formats suitable for automated deposition into databases for processing and visualization. HCS—image files and calculated parameters and metadata... 

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