Bacterial Colonies in Solid Media and Foods: A Review on Their Growth and Interactions with the Micro-Environment

Abstract

Bacteria, either indigenous or added, are immobilized in solid foods where they grow as colonies. Since the 80's, relatively few research groups have explored the implications of bacteria growing as colonies and mostly focused on pathogens in large colonies on agar/gelatine media. It is only recently that high resolution imaging techniques and biophysical characterization techniques increased the understanding of the growth of bacterial colonies, for different sizes of colonies, at the microscopic level and even down to the molecular level. This review covers the studies on bacterial colony growth in agar or gelatine media mimicking the food environment and in model cheese. The following conclusions have been brought to light. Firstly, under unfavorable conditions, mimicking food conditions, the immobilization of bacteria always constrains their growth in comparison with planktonic growth and increases the sensibility of bacteria to environmental stresses. Secondly, the spatial distribution describes both the distance between colonies and the size of the colonies as a function of the initial level of population. By studying the literature, we concluded that there systematically exists a threshold that distinguishes micro-colonies (radius < 100–200 μm) from macro-colonies (radius >200 μm). Micro-colonies growth resembles planktonic growth and no pH microgradients could be observed. Macro-colonies growth is slower than planktonic growth and pH microgradients could be observed in and around them due to diffusion limitations which occur around, but also inside the macro-colonies. Diffusion limitations of milk proteins have been demonstrated in a model cheese around and in the bacterial colonies. In conclusion, the impact of immobilization is predominant for macro-colonies in comparison with micro-colonies. However, the interaction between the colonies and the food matrix itself remains to be further investigated at the microscopic scale.

Characterisation of Aerotolerant Forms of a Robust Chicken Colonizing Campylobacter coli

“…Growth on solid medium creates oxygen gradients, such that facultative anaerobes have been found to be more active on the bottom layer and aerobes more rapidly dividing on the top layer of colonies (Reyrolle and Letellier, 1979). Oxygen gradients in bacterial colonies and solid media have also been measured analytically, with the top 30 μm of a gelatine medium being considered aerobic (Jeanson et al, 2015). Therefore, once initial growth of Campylobacter has commenced on a solid medium, bacteria that are not directly exposed to the outside atmosphere may reside in an atmosphere more similar to microaerobic conditions.…”

New insight on crystal and spot development in hard and extra-hard cheeses: Association of spots with incomplete aggregation of curd granules

“…In the young cheese, even before the spot formation, these hollow cavities likely represent preferential zones where whey stagnates and entrapped bacterial cells find substrates for growth (Le Boucher et al, 2016). Growth of bacterial colonies as affected by local concentration of substrates in cheese is receiving increasing attention (Silva et al, 2013;Jeanson et al, 2015). Although no intact bacterial cells were detected by TEM in cavities within the spot or in the surrounding cheese, likely due to the prolonged ripening process, an indirect, preliminary confirmation of this hypothesis came from measuring the amount of total DNA extracted from the spot and the surrounding cheese following the procedure of Cremonesi et al (2007).…”

Overproduction of individual gas vesicle proteins perturbs flotation, antibiotic production and cell division in the enterobacterium Serratia sp. ATCC 39006

“…However, microbes are known to grow differently on agar plates than in liquid medium (Jeanson et al, 2015;Mikkelsen et al, 2007). As gas vesicles facilitate flotation in aquatic environments, we also wanted to examine gas vesicle production in liquid culture.…”