A method for determining the presence of at least one determined microorganism in a Petri dish comprising one or more colonies of microorganisms and a culture medium, said culture medium being adapted to allow the one or more colonies of microorganisms and said at least one determined microorganism, if present, to grow under suitable growth conditions, the method comprising: - obtaining using an image system at least an initial image of the Petri dish, wherein the first initial image comprises one or more visible pixels, each pixel being associated with a pixel value; - obtaining a first processed image of the Petri dish by applying a first process to the at least one initial image, wherein visible pixels of the first processed image only relate to the pixels associated with the one or more colonies of microorganisms; - obtaining a plurality of second processed images of the Petri dish by applying a second process to the first processed image, wherein visible pixels of the second processed image only relate to the pixels associated with the one or more colonies of microorganisms and a surrounding zone around said one or more colonies of microorganisms; - obtaining a plurality of third processed images of the Petri dish by calculating the difference between the at least one initial image and the plurality of second processed images, wherein the plurality of third processed images comprises visible pixels which only relates to pixels associated with the culture medium; - obtaining a plurality of fourth processed images of the Petri dish by calculating the difference between the plurality of second processed images and the first processed image, wherein visible pixels of the plurality of fourth processed images only relate to pixels associated with the surrounding zone; - determining features values associated with the plurality of the third and the fourth processed images by calculating the difference values between the average pixel values of the plurality of the third processed image and the average pixel values of the plurality of the fourth processed image for at least each colour channel of the Red Green Blue colour channels; - determining a value of an indicator of the presence of the at least one determined microorganism within the surrounding zone in the Petri dish by classifying the determined features values; - comparing the value of the indicator with a threshold value; - depending on the result of the comparison, determining the presence of the at least one determined microorganism within the surrounding zone in the Petri dish. - obtaining using an image system at least an initial image of the Petri dish, wherein the first initial image comprises one or more visible pixels, each pixel being associated with a pixel value; - obtaining a first processed image of the Petri dish by applying a first process to the at least one initial image, wherein visible pixels of the first processed image only relate to the pixels associated with the one or more colonies of microorganisms; - obtaining a plurality of second processed images of the Petri dish by applying a second process to the first processed image, wherein visible pixels of the second processed image only relate to the pixels associated with the one or more colonies of microorganisms and a surrounding zone around said one or more colonies of microorganisms; - obtaining a plurality of third processed images of the Petri dish by calculating the difference between the at least one initial image and the plurality of second processed images, wherein the plurality of third processed images comprises visible pixels which only relates to pixels associated with the culture medium; - obtaining a plurality of fourth processed images of the Petri dish by calculating the difference between the plurality of second processed images and the first processed image, wherein visible pixels of the plurality of fourth processed images only relate to pixels associated with the surrounding zone; - determining features values associated with the plurality of the third and the fourth processed images by calculating the difference values between the average pixel values of the plurality of the third processed image and the average pixel values of the plurality of the fourth processed image for at least each colour channel of the Red Green Blue colour channels; - determining a value of an indicator of the presence of the at least one determined microorganism within the surrounding zone in the Petri dish by classifying the determined features values; - comparing the value of the indicator with a threshold value; - depending on the result of the comparison, determining the presence of the at least one determined microorganism within the surrounding zone in the Petri dish.