Lactobacillus backi – A Beer Spoiler

Lactobacillus backi is a bacteria belonging to the Lactobacillus genus responsible for spoiling beer in brewing industries.  

Lactobacillus species are known to spoil beer and are therefore causing a significant financial impact on the brewing industries. Research has been conducted to identify the bacteria responsible for causing beer spoilage. One such bacteria identified is the Lactobacillus backi strain of bacteria from the Lactobacillus genus. Therefore, this bacteria produces lactic acid as a byproduct belonging to the Lactobacillus genre. Additionally,  Werner Back (classification in brewing microbiology) identified this bacteria, hence the name Lactobacillus backi. One research study subdivided beer spoiling bacteria into two categories; obligate beer-spoiling bacteria and potential beer-spoiling bacteria. Aside from Lactobacillus backi, other bacterias are involved in beer spoilage, such as Lactobacillus brevis, Lactobacillus casei, Lactobacillus coryniformis, etc. Therefore, the beer industry needs to control the conditions for preparing and fermenting beer to prevent any bacterial growth in beer. 

Overview of Lactobacillus backi

Lactobacillus backi is an irregular rod-shaped, non-motile, non-spore-forming, and gram-positive bacteria. These bacteria do not contain catalase, and therefore they are not capable of catalysing hydrogen peroxide (H2O2) to form oxygen and water as byproducts. At the microscopic level, these bacteria cells are present in a single form, in pairs, and sometimes in a chain form. These bacteria are facultatively anaerobic, and their optimal growth temperature is 28°C – 36°C with a pH of 4.5 to 6.5. The bacterial colony is observed to be flat or slightly exalted, smooth, and white colonies. This bacteria generates only a single byproduct, lactic acid, after metabolising glucose substitutes; therefore, they are homofermentative. However, no gas is produced from glucose metabolism upon lactic acid release. 

Since there are multiple beer-spoiling bacteria, physiological tests are carried out to determine the presence of Lactobacillus backi. These tests include the determination of D-lactic acid and L-lactic acid enzymatically. 16sRNA gene is isolated from Lactobacillus backi to assess their specific sequences for phylogenetic analysis (to identify the evolution of the species). Lactobacillus backi species is morphologically similar to Lactobacillus coryniformis. The primary characteristic of Lactobacillus backi is their ability to hydrolyse aesculin and ferment glucose substitutes. 

How Lactobacillus backi  spoils beer 

When brewing beer, under a pH of 3.8-4.7, only a restricted amount of bacteria can grow in beer due to alcohol. The beer is brewed under anaerobic conditions and lacks nutrition for the antibacterial effects of the hop plant (Humulus lupulus from the genus Humulus). The bacteria that are responsible for spoiling beer belong to the lactobacillus genre. Therefore, it has become an essential factor in determining the brewing conditions that cause these bacterial growths. Since each bacteria has its optimal growth conditions, detecting the species of bacteria will help in altering the optimal environment required by the bacteria to grow. However, methodologies to detect the species-specific bacteria are difficult to establish as there is a wide variety of bacteria present. 

Beer-spoiling bacteria can be subdivided into obligate beer bacteria and potential beer-spoiling bacteria. Some beer spoiling bacterias can rapidly deteriorate the beer product, thereby displaying high bacterial capability for beer spoilage, regardless of the beer type. In comparison, potential beer-spoiling bacteria are dependent on the beer conditions and its ingredients, in addition to the ability of the bacteria to adjust to these conditions. Despite beers having specific antimicrobial resistance, however, they can still undergo bacterial spoilage that leads to a decrease in pH value, sedimentation, and turbidity. Sedimentations are protein and yeast particles found at the bottom of the bottle, caused due during the brewing process or poor filtration. Turbidity in the beer can indicate the presence of microbes or other particles that could be harmful, causing the liquid to become hazy. The finished beer product should not contain any cloudy liquid. Instead, it should be clear and non-hazy to show the beer as a good quality product.