Plate Count

Plate Count of bacteria and funghi in drainwater

The Interpretation of Total Plate Count of Bacteria and fungi in water samples.

Just recently we have developed a new analysis package called Plate Count Concentrated with the aim to determine the amount viable spores of 3 important pathogenic fungi in (drain) water. To determine the viable spores of fungi it is better to test by plate count methods then using methods based on detection of DNA. In this way only the living spores of these fungi are determined instead of both living and death spores (DNA Method). Disinfected water is water which is treated by a disinfection technique (f.i. UV-treatment) or treated with a disinfectant. In analysis package Plate Count Concentrated we measure the general plate count of bacteria & fungi, Fusarium spp, Pythium spp. and Phytophtora spp. Because these plant pathogens can be present in low concentrations in water, the sample is first concentrated before the actual analysis is performed. In this way it is possible to determine traces of these fungi in even lower concentrations and it is possible to check whether the efficacy of the disinfection process in removing these fungal pathogens.

The illustration below depicts an example of the analysis report of Plate Count Concentrated; general plate count of bacteria and fungi are reported in CFU/ml. Pythium spp. and Phytophtora spp are determined as present (+) or not present (-) in 100 ml. Fusarium spp. is reported in  CFU in 50 ml, a value of <1 means that Fusarium is not present in 50 ml.

We perform a lot of microbiological analyses on water samples that are taken before and after UV-disinfection. To obtain representative analysis results a number of important aspects should be taken into account:

  • Sampling should occur by a special hygienic protocol.
  • Use sterile bottles (with a red lid). This is highly important because regular water bottles can already contain traces of micro-organisms.
  • The sample should be taken while the UV-installation is running.
  • Sample transport in cold atmosphere.
  • The laboratory analysis should start within 12 hours after sampling. 

The number of bacteria and fungi in drainwater. In 'dirty' water in horticulture up to 3 million (=Log6) colony forming units (cfu) per ml can be present. For fungi these values can be up to 800.000 cfu per ml. These values should be interpreted in the right manner especially when these analyses are used to measure the efficiency of disinfection. For example a value of 200.000 CFU/ml for total plate count of bacteria is not that much lower then 500.000 CFU/ml. The starting point is that 1 micro-organism grows into 1 colony on a specific medium. Despite the fact that no clear standards exist for disinfected drain water, it is desired that the number of bacteria after disinfection is zero or close to zero. The general aim is that disinfected water should show a 99,9 % reduction in bacterial plate count compared to non-disinfected water. Unfortunately this is not always the case.

The amount of bacteria after disinfection? In terms of disinfection, one speaks about a so-called Log-3 reduction meaning that 1 in 1000 bacteria will survive the disinfection treatment. Growers tend to see a total plate count value of 500.000 CFU/ml as much higher than a plate count of 200.000 CFU/ml. But in fact these values should be considered as more or less the same as they are in the same order of magnitude. Drainwater of good quality should not contain a total bacterial plate count over 1500 CFU/ml. After disinfection this value should be much lower to reduce the number of regrowth of bacteria while this water is stored in the watersilo. For this reason it is recommended to reduce the storage time and make sure that the storage conditions are cool and dark. In addition this silo should be cleaned at least once a year. 

The amount of fungi after disinfection. In general fungi are harder to kill compared to bacteria. This is why fungi can be still present after UV-disinfection, while the amount of bacteria is zero or close to zero.

The analysis package: ''plate count concentrated'' is also suitable to determine the number of bacteria or fungi present in cuttings or in water originating from seed boxes. In this way, monitoring of bacterial and fungal plate counts including the presence of Pythium spp, Phytopthora spp. and Fusarium spp. can start early in the supply chain because it is already measured in cuttings. In case of high plate count values or presence of these pathogenic fungi one can take action. In addition some quality certificates like GlobalGAP or QS ask for specific analyses on process water like E.coli and Faecal enterococci. These are different analyses, so it is important to ask for the right analysis.