Chapter 16, Problem 1AQ

Enteric bacteria, lactic acid bacteria, and propionic acid bacteria have distinctive metabolic traits that can be used to characterize and identify these organisms. Describe the metabolic characteristics of these organisms, name a genus that belongs to each group, and indicate in what way these organisms can be differentiated.

Summary Introduction

To describe:

The metabolic characteristics of enteric bacteria, propionic acid bacteria, and lactic acid bacteria, name a genus that belongs to each group, and indicate by what means these organisms can be distinguished.

Concept introduction:

The order Lactobacillales comprised of fermentative organisms such as lactic acid bacteria. The lactic acid bacteria generate lactic acid as the end-product after the fermentation process. Enteric acid bacteria are bacteria usually resides in the intestine of humans and animals. The enteric bacteria play an important role in digestion of their hosts. The propionic acid bacteria reside in the sweat glands of the human and it is also present in the stomach of the ruminants.

Explanation of Solution

Enteric bacteria, for example species of Escherichia are facultative anaerobes. They inhabit the gastrointestinal tract of humans and warm-blooded animals, where they characteristically grow chemoorganotrophically by means of fermentation. They live in a wide range of substrates. Under anaerobic conditions they use mixed-acid fermentation generating succinate, acetate, ethanol, and lactate. 

Lactic acid bacteria, for example species of Lactobacillus are aerotolerant, anaerobes. During fermentative metabolism, they yield lactate as the major product, in which energy is produced by substrate-level phosphorylation. The lactic acid bacteria have restricted biosynthetic abilities and characteristically have complex nutritional necessities such as amino acids, vitamins, purines, and pyrimidines.

Propionic acid bacteria are fermentative bacteria, for example species of Propionibacterium. They commonly establish in the sweat glands of humans and the stomachs of ruminants. The propionic acid bacteria generate acetic acid, propionic acid, and Co₂ as products of anaerobic metabolism. Lactate is the major substrate for propionic acid bacteria, which is already a fermentation product. Therefore, propionic acid bacteria carry out secondary fermentation by fermenting the previous product of fermentation.