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

Jeanson S, Floury J, Gagnaire V, Lortal S, Thierry A - Front Microbiol (2015)

Bottom Line: The following conclusions have been brought to light.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).In conclusion, the impact of immobilization is predominant for macro-colonies in comparison with micro-colonies.

View Article: PubMed Central - PubMed

Affiliation: INRA, UMR1253, Science and Technology of Milk and Eggs Rennes, France ; AGROCAMPUS OUEST, UMR1253, Science and Technology of Milk and Eggs Rennes, France.

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.

No MeSH data available.


Related in: MedlinePlus

pH profiles measured using a pH-sensitive fluorophore (C-Snarf-4) and confocal microscopy for a colony (radius = 65 μm) growing in a model cheese throughout acidification: 19 h (), 24 h (), 26 h (), and all measurements from 42 to 72 h (red bold line, ). Adapted from Jeanson et al. (2013).
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Figure 7: pH profiles measured using a pH-sensitive fluorophore (C-Snarf-4) and confocal microscopy for a colony (radius = 65 μm) growing in a model cheese throughout acidification: 19 h (), 24 h (), 26 h (), and all measurements from 42 to 72 h (red bold line, ). Adapted from Jeanson et al. (2013).

Mentions: In order to confront the observations in agar and gelatine to a real food medium, pH was measured at the microscopic level in a model cheese and in real commercial cheeses. Using ratio-imaging fluorescence, local pH was measured during the acidification of colonies of L. lactis whose radii ranged from 17.5 to 55.5 μm, corresponding to the lowest inoculation levels possible in cheesemaking, ranging from 1.3 × 103 to 1.6 × 105 cfu/ml, respectively (Jeanson et al., 2013). Regardless of the observed colony size, no pH microgradients could be observed around colonies (Figure 7). Furthermore, in the same model cheese, the same strain of L. lactis displayed no evidence of acid stress at the gene expression level (Cretenet et al., 2011). These results are in agreement with those described above and observed in a gelatine medium for colonies of L. curvatus up to 150 μm (Malakar et al., 2000). These consistent results demonstrate that the diffusion of lactic acid was not the limiting factor for growth neither in gelatine nor in a model cheese containing colonies which radius was smaller than 150 μm. Furthermore, in ripened commercial Cheddar cheeses, pH microgradients have been observed at the microscopic scale of a few μm using the fluorescence life-time (FLIM), but not especially around colonies (Burdikova et al., 2015). The accumulation of lactic acid around the colonies has been suggested as the main explanation for the lower growth rate in renneted milk gels when compared with that in liquid milk (Stulova et al., 2015). The simplified composition (no fat, no NaCl) and the homogeneous structure of the model cheese (Jeanson et al., 2013) may explain the non-accumulation of lactic acid around small colonies whilst in commercially available cheeses (Burdikova et al., 2015), lactic acid concentration may vary at the microscopic scale because of a more heterogeneous microstructure.


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

Jeanson S, Floury J, Gagnaire V, Lortal S, Thierry A - Front Microbiol (2015)

pH profiles measured using a pH-sensitive fluorophore (C-Snarf-4) and confocal microscopy for a colony (radius = 65 μm) growing in a model cheese throughout acidification: 19 h (), 24 h (), 26 h (), and all measurements from 42 to 72 h (red bold line, ). Adapted from Jeanson et al. (2013).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4664638&req=5

Figure 7: pH profiles measured using a pH-sensitive fluorophore (C-Snarf-4) and confocal microscopy for a colony (radius = 65 μm) growing in a model cheese throughout acidification: 19 h (), 24 h (), 26 h (), and all measurements from 42 to 72 h (red bold line, ). Adapted from Jeanson et al. (2013).
Mentions: In order to confront the observations in agar and gelatine to a real food medium, pH was measured at the microscopic level in a model cheese and in real commercial cheeses. Using ratio-imaging fluorescence, local pH was measured during the acidification of colonies of L. lactis whose radii ranged from 17.5 to 55.5 μm, corresponding to the lowest inoculation levels possible in cheesemaking, ranging from 1.3 × 103 to 1.6 × 105 cfu/ml, respectively (Jeanson et al., 2013). Regardless of the observed colony size, no pH microgradients could be observed around colonies (Figure 7). Furthermore, in the same model cheese, the same strain of L. lactis displayed no evidence of acid stress at the gene expression level (Cretenet et al., 2011). These results are in agreement with those described above and observed in a gelatine medium for colonies of L. curvatus up to 150 μm (Malakar et al., 2000). These consistent results demonstrate that the diffusion of lactic acid was not the limiting factor for growth neither in gelatine nor in a model cheese containing colonies which radius was smaller than 150 μm. Furthermore, in ripened commercial Cheddar cheeses, pH microgradients have been observed at the microscopic scale of a few μm using the fluorescence life-time (FLIM), but not especially around colonies (Burdikova et al., 2015). The accumulation of lactic acid around the colonies has been suggested as the main explanation for the lower growth rate in renneted milk gels when compared with that in liquid milk (Stulova et al., 2015). The simplified composition (no fat, no NaCl) and the homogeneous structure of the model cheese (Jeanson et al., 2013) may explain the non-accumulation of lactic acid around small colonies whilst in commercially available cheeses (Burdikova et al., 2015), lactic acid concentration may vary at the microscopic scale because of a more heterogeneous microstructure.

Bottom Line: The following conclusions have been brought to light.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).In conclusion, the impact of immobilization is predominant for macro-colonies in comparison with micro-colonies.

View Article: PubMed Central - PubMed

Affiliation: INRA, UMR1253, Science and Technology of Milk and Eggs Rennes, France ; AGROCAMPUS OUEST, UMR1253, Science and Technology of Milk and Eggs Rennes, France.

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.

No MeSH data available.


Related in: MedlinePlus