What's the purpose of using more than one culture medium for bacterial isolation?

Question

What's the purpose of using more than culture medium for bacterial isolation?

Why not use just one?

If this is not generally used in isolation of various types of bacteria, I'm concerned especially with Acinetobacter spp. isolation, which grow well on blood agar and MacConkey agar.

Examples of research which include the isolation of acinetobacter spp.:

In this study, 200 (9.5%) Gram-negative, non-fermentative strains were selected among 2108 Gram-negative isolates identified at mega laboratories over the Suez Canal and Nile Delta regions in Egypt (1044 urine culture, 108 pus, 537 sputum, 322 blood, 23 cerebrospinal fluid, and 74 wound) from August 2017 to October 2018. The 200 strains were preliminary inoculated on MacConkey agar, blood agar, Acinetobacter agar, and CHROM agar media. [1]

The samples were collected from patients of all age groups, both sexes, who were critically ill and suspected for pneumonia, urinary tract infection, septicaemia, skin and soft tissue infection and meningitis. The samples were inoculated on Blood Agar and MacConkey Agar plates under strict aseptic conditions. [2]

All the samples including bland pus, respiratory tract samples, fluids, swabs and urine samples were surface streaked onto BA [Blood agar] and MacConkey agar plates. [3]

1. https://doi.org/10.3390/antibiotics9090603
2. Oberoi, A., Aggarwal, A., & Lal, M. (2009). A decade of an underestimated nosocomial pathogen - Acinetobacter in a tertiary care hospital in Punjab.
3. https://doi.org/10.22159/ajpcr.2018.v11s2.28590

Edit: There is one species in the genus Acinetobacter, Acinetobacter haemolyticus, which causes beta hemolysis on blood agar, however, not a single research that mentions using both MacConkey and blood agar, mentions this as the reason for using blood agar.

Answer 1

I am not sure I get your question, Do you want to isolate a sepcific type of bacterium or in general isolate most bacteria from a sample?

If you want to isolated a specific bacterium from a samples, there various "selective" media you can use. Now, I never saw a medium that is really selective, meaning other stuff will grow on it as well. But in general you can be confident that the diversity of colonies you get on these media is lower than what you get on a general medium.

If you want to isolate as many bugs as you can, well then you have to try as many media as you can. Not all bugs will like the same medium. Also there is this misconception that bacteria like rich media. This is not necessarily the case and often rich media are actually highly selective for fast growing bugs that are able to tolerate well stress, carbon overflow, and ROS/NOS.

Answer 2

This is a great question! We often use multiple media types for isolation in microbiology because microbes have widely different growth needs. Using various media types, we can create more than one growing environment and capture a wider range of the microbial population. Additionally, different media can inform us about the microbial population's biochemistry. For example, blood agar plates can be used to test for hemolysis capabilities. There are a lot of media types so the purpose of each one is unique. If you are interested in learning about more specific media types I would suggest reviewing the research and identifying the main research questions. Important note on microbial culturing: there is a large percentage of microbes that cannot be currently cultured through traditional culturing techniques. Microbiologists are advancing culturing techniques and increasing our ability to grow fastidious, difficult-to-grow, organisms. However, this is co-occurring with a greater understanding of the massive amount of microbial diversity that exists in the environment. Luckily, we now have culture-independent methods to identify microbial community populations.