Nature de la flore microbienne dominante et sous-dominante des laits crus très pollués

Abstract - Composition of dominant and subdominant flora of milk of poor bacteriological quality
The composition of the dominant and subdominant microbial flora of raw milk produced under different conditions (hand or machine milking, milk storage at three ranges of temperature i.e. < 5° C, 8 -12° C, ≥ 15° C for 1 or 2 days) has been studied at 3 periods in the course of one year (spring, summer and autumn).
Samples dilutions (1:50 in quarter strength Ringer solution) were made and mechanically shaken to break down the clumps before they were surface plated on Plate count Agar (PCA) and Nutrient Agar containing 100 IU / mL penicillin, using a Spiral Plater. After incubation for 1-2 d at 30° C plus 2-3 d at room temperature the large (>2 mm) and the "small" (≤2 mm) colonies on PCA were counted using a laser counter adapted to the Spiral plating.
From each plate for total count 25 large and, when ever possible, 8 small well isolated colonies were picked at random and without any purification transferred on various test media for primary characterization; large colonies were submitted to 10-16 tests; these were chosen to demonstrate technological properties of isolates (growth at low temperature, lipolysis, proteolysis, production of acid from lactose) and included taxomonical tests easy to perform on a large scale. The criteria for small colonies were those in use for the identification of the lactic streptococci and the separation of the other streptococci into genus or groups. A representative collection of 182 penicillin resistant bacteria was purified and submitted to an adansonian classification based on 32 physiological and biochemical criteria including thoses for primary characterization. A set of 7 criteria was derived from this step of the study to try to identify the remaining isolates of penicillin resistant bacteria.
On the whole, 4810 large colonies from 231 milk samples were characterized. Most of them were resistant to penicillin; in this group coliform bacteria and Pseudomonas (fluorescent group) were dominant Acinetobacter spp. and yellow pigmented bacteria allocated to Cytophaga-Flavobacter group were present at a lower level. The penicillin sensitive bacteria were micrococci, coryneform bacteria and some coliform and yellow pigmented bacteria, the whole showing a weak biochemical pattern as compared to Pseudomonas and coliform bacteria. Most of the small colonies (1424 strains examined, from 178 samples) were streptococci, about 50 % being identified as S. lactis-cremoris. The genus Leuconostoc was present at a significant level (10-20 %) only in autumn; fecal streptococci were at a very low level.
No evidence of a competition between the lactic acid bacteria and the Pseudomonas counts in the milk was observed. It was concluded that the lactic streptococci had no action on the undesirable flora of the raw milk, probably because they did not reach a sufficient level of population

Résumé - Pour étudier la nature de la flore microbienne dominante et sous dominante du lait cru très pollué (entre 10⁵ et 10⁸ microorganismes / mL), la flore totale et la flore résistante à la pénicilline de 231 échantillons de diverses origines (lait de traite manuelle ou mécanique, refroidi ou non à la ferme) ont été évaluées par ensemencement à la surface de la gélose à l'aide d'un appareil " Spiral ".
A partir des boîtes de dénombrement de la flore totale, on a prélevé 25 grosses colonies par échantillon et, lorsqu'elles étaient en flore dominante ou sous dominante, 8 petites colonies. Les premières ont été caractérisées à l'aide de 4 critères d'intérêt technologique (croissance au froid, lipolyse, protéolyse, production d'acide à partir du lactose) et de 10 à 12 critères taxonomiques ; les secondes sur la base de 6 critères choisis pour identifier les streptocoques les plus intéressants. Une collection de 182 souches représentatives des grosses colonies résistantes à la pénicilline, a fait l'objet d'une classification numérique sur la base de 32 critères biochimiques et physiologiques.
Les grosses colonies ont été séparées en deux groupes sur la base de leur résistance à la pénicilline. Celles qui étaient résistantes (les plus nombreuses) étaient composées principalement de bactéries coliformes et de Pseudomonas du groupe fluorescent. Les colonies sensibles comprenaient aussi des bactéries à Gram négatif, mais surtout des microcoques et des bactéries corynéformes ; dans l'ensemble elles présentaient des caractères technologiques peu marqués. Les petites colonies étaient le plus souvent des streptocoques dont la moitié identifiée à S. lactis-cremoris.
Aucune compétition entre la flore lactique et la flore nuisible (Pseudomonas ou bactéries coliformes) n'a été mise en évidence