DDC Tech Day | Antimicrobial Function of Acidifiers and Comparison of Antibacterial Effects Among Common Acid Ingredients

Abstract: Currently, many countries around the world are exploring antibiotic alternatives that can promote animal growth. Acidifiers are widely regarded as the most commonly used and effective alternative to antibiotics. Numerous studies have shown that adding acidifiers to feed can enhance animal productivity, improve feed efficiency, safeguard the health of the gastrointestinal mucosa, stimulate the growth of beneficial gut bacteria, inhibit the proliferation of harmful microorganisms, and boost animals' immune function. This article provides a review of the antibacterial mechanisms of acidifiers, as well as a comparative analysis of the antimicrobial properties of common acidic ingredients.

Keywords: Acidifying agent; acid raw material; antibacterial effect

Adding acidifiers to feed can reduce the gastrointestinal tract of animals. pH It works by altering the gut microbiome's environment, inhibiting the growth and reproduction of harmful bacteria—and even directly killing them—while simultaneously promoting the proliferation of beneficial probiotics. This is because pathogenic bacteria such as E. coli and Staphylococcus thrive most effectively in the pig intestine. pH Value is 6.0–7.5 , less than 4  This can inhibit its growth while promoting the growth and reproduction of beneficial probiotic bacteria, such as intestinal lactobacilli.

Adding different acidifiers can all promote piglet growth to varying degrees and reduce the incidence of diarrhea; among them 1.5 ％Citric acid significantly reduces the number of E. coli in the colon and lowers the incidence of diarrhea; phosphate, however, proves less effective than both citric acid and the compound acid. Most researchers believe that dietary acidification can effectively control harmful bacterial populations in the piglets' digestive tract. ( Escherichia coli, Salmonella, Staphylococcus, Clostridium perfringens ) Quantity. Reducing the diarrhea rate in piglets, as dietary acidifiers lower intestinal pH , disrupting the favorable environment for harmful microorganisms while simultaneously creating an ideal habitat for beneficial bacteria like lactic acid bacteria to thrive and multiply. The metabolic byproducts of lactic acid bacteria—such as lactic acid— H2O2 It also exhibits inhibitory and lethal effects on bacteria such as E. coli. Therefore, acidifiers will help optimize the structure of the intestinal microbiota, reduce pathogen infections, and create a healthy gut environment for animals.

1 On the Antimicrobial and Fungicidal Effects of Acidifiers

Yang Fulin's research indicates that adding [something] to the diet of weaned piglets 1.7 % and 3.3 Compared to the control group, the number of E. coli in the cecum and colon of piglets was reduced. 91 %, 8l % and 95 %, 87 %, the number of Lactobacillus increased respectively 95 %, 86 % and 97 %, 92 %, the average diarrhea rate in piglets was reduced respectively. 60 % and 64 ％。This indicates that acidifiers not only inhibit pathogenic bacteria but also protect beneficial gut microbiota. Lin Yingcai and colleagues' research suggests that adding acidifiers to the diet 1500g ／ t Compound acid, primarily based on phosphoric acid, combined with citric acid, lactic acid, and fumaric acid, helps piglets 4 ～ 7kg The number of Lactobacillus in the small intestine, the number of Escherichia coli, and the total bacterial count all tended to increase. Notably, among the total bacteria, the proportion of Lactobacillus rose, while the proportion of Escherichia coli declined, indicating that the appropriately formulated compound acid not only directly influences the growth of intestinal bacteria in piglets but, more importantly, helps maintain the balance of the gut microbiota.

1.1 Mechanism of Antimicrobial Action of Organic Acids

One way organic acids exert their antibacterial effects is by releasing hydrogen ions to lower the pH of the digestive tract. pH Thus, disrupting the favorable living environment of harmful bacteria. Another approach involves undissociated organic acid molecules penetrating the cell walls of Gram-negative bacteria and entering the cells, where these organic acid molecules then encounter the relatively alkaline intracellular environment inside the bacterial cells. pH = 7.4 ～ 7.5 It dissociates into hydrogen ions and anion radicals, with hydrogen ions lowering the intracellular environment. pH , while the cells strive to restore themselves pH Balancing and releasing protons depletes the cell's own energy, leading to significant energy loss. Additionally, the accumulation of acid-derived ions can directly interfere with and block processes within the cell nucleus. DNA The synthesis; this dual mechanism of organic acids effectively inhibits bacterial proliferation.

1.2 Acids with different degrees of dissociation have distinct physiological effects.

The acids commonly used as acidifiers include citric acid, fumaric acid, lactic acid, phosphoric acid, formic acid, propionic acid, butyric acid, and their respective salts. Studies have shown that the effectiveness of these acids in acidifying feed varies, with formic acid ranking the lowest in terms of acidification potency. > Phosphoric acid > Tartaric acid > Malic acid = Citric acid > Lactic acid > Acetic acid. The effect of adding acid to feed lies, on one hand, in the fact that acid can lower the pH of the diet. pH On the other hand, this is also thanks to the antibacterial effect of acids. Acids with different degrees of dissociation exhibit distinct effects. Each acid possesses its own unique pK Value, whose negative value is acidic. 50 ％When dissociating pH Value, pK The larger the value, the less likely the acid is to dissociate. For example, formic acid's pK Value is 3.77 , while propionic acid's pK Value is 4.88 , they exhibit strong bactericidal and antibacterial effects in applications. Phosphoric acid has three pK Value, its smallest pK Value is 1.96 Therefore, compared to organic acids, it has a relatively higher degree of dissociation, making it particularly effective in reducing the digestive tract when used in combination with compound acidifiers. pH The effect is remarkably significant. Based on the theory of varying dissociation degrees in acidifiers and their synergistic antibacterial action, one can select acids with high dissociation degrees to achieve a reduction. pH . Properly balanced organic acids can fully leverage the complementary and synergistic effects among different acids, thereby reducing issues in the digestive tract. pH It improves the physiological environment of the digestive tract and also exerts effective antibacterial and bactericidal effects, enhancing the ecological balance of the digestive system—effects that have been proven superior to those of single acids, as confirmed by numerous studies.

1.3 Volatile Organic Acids

Research has found that there are significant differences in the antibacterial effects of volatile organic acids, non-volatile organic acids, and inorganic acids. Volatile organic acids—such as formic acid, propionic acid, and butyric acid—demonstrate superior bactericidal activity, as they can directly diffuse into cells, leading to a reduction in intracellular concentrations. pH Value, inhibiting certain macromolecules such as DNA 、 RNA The metabolism of cellular membrane components such as proteins or lipids disrupts the integrity of bacterial cell membranes, thereby achieving a bactericidal effect. Although formic acid and other compounds exhibit pronounced antibacterial activity, numerous studies also indicate that formic acid—or even mixtures of formic and propionic acids—can significantly reduce feed intake. This is because certain acids like acetic acid, propionic acid, formic acid, and lactic acid become more inhibitory when present in neutral conditions. pH In the environment, it only partially dissociates and can penetrate cell membranes, acidifying the neuron cytoplasm and stimulating nociceptors, thereby triggering sensations of irritation and pain. However, other strong acids such as citric acid and tartaric acid elicit less sensitive responses at the trigeminal nerve level, failing to induce—or producing only subtle—stinging sensations. Wang et al., 2011 ). Therefore, the use of acetic acid, propionic acid, formic acid, and lactic acid may potentially trigger aversive responses. However, the potassium salt of formic acid did not elicit any noticeable aversion. In fact, a significant portion of the market has already shifted from using organic acids to organic acid salts, which indirectly highlights the irritant effects of weak acids on pigs and their subsequent suppression of feed intake.

Both non-volatile organic acids and inorganic acids exhibit significant antibacterial effects. These acidifiers can reduce the bacterial extracellular matrix pH It disrupts the osmotic pressure system, preventing cells from reproducing normally, thereby inhibiting bacterial growth and proliferation.

The primary action sites of organic acids are the feed itself and the stomach; by the time organic acids reach the small intestine, they become nearly neutral. pH The rapid absorption mechanism weakens the antibacterial ability of organic acids in this part of the digestive tract. Data indicate that sorbic acid exhibits strong antimicrobial effects. , The effects of acetic acid and fumaric acid are secondary. Meanwhile, other organic acids such as malic acid, citric acid, and lactic acid have very limited effectiveness against Salmonella and other pathogenic bacteria.

1.4 The Antimicrobial Effect of Volatile Organic Acids

In addition to organic and inorganic acids, volatile organic acid salts also exhibit antibacterial activity; however, these acid salts are unable to penetrate the bacterial cell envelope. pH It cannot penetrate bacterial cells in its acidic form to exert its effect; instead, its potency depends on the solubility of its salt. For instance, since sodium salts of organic acids are more soluble than calcium salts, sodium butyrate demonstrated stronger antibacterial activity in vitro compared to calcium formate and calcium propionate.

1.5 The Antimicrobial Effect of Methionine Hydroxy Analogues

Methionine hydroxy analog HMT-BA ） 88 % Aqueous Solution pH Slightly smaller than 1 , which may share similar antibacterial mechanisms with organic acids such as formic acid, lactic acid, and citric acid. In vitro antibacterial assay results can indicate the sensitivity level of pathogenic bacteria to the antimicrobial agents, providing reference data for screening the types and dosages of antimicrobials in animal experiments. The study found HMT-BA At 12.50 mmol ／ L Equivalent to mass fraction 0.21 At certain concentrations, it can inhibit the growth and reproduction of pathogenic bacteria such as Salmonella pullorum and Salmonella enteritidis; HMT-BA At 25.00 mmol ／ L Equivalent to mass fraction 0.42 It can inhibit the growth and reproduction of Clostridium perfringens and Campylobacter jejuni. In broiler production, the typical dosage of methionine hydroxy analogues is generally between 0.20 %～ 0.30 Approximately %. Based on this, it is speculated that, in addition to supplementing methionine, the methionine hydroxy analog may exhibit certain antibacterial effects during feed storage or within the gastrointestinal tract of livestock and poultry. However, the exact nature of these effects still requires further verification through animal trials. HMTBA To 4 The in vitro antibacterial efficacy of the tested strain was comparable to that of equimolar formic acid and potassium diformate, yet higher than that of equimolar lactic acid. In vitro 12.50 ～ 25.00 mmol/L HMTBA It can inhibit the growth and reproduction of pathogenic bacteria.

2 Acidifiers promote the growth and reproduction of probiotics

Adding acidifiers to feed lowers the acidity in the gastrointestinal tract, creating an environment conducive to the growth of beneficial probiotic bacteria such as lactobacilli. However, different types of acids or acid salts have varying effects on probiotics. Reports indicate that certain organic acids—such as lactic acid, citric acid, and fumaric acid—which can serve as intermediates in metabolic pathways—actually enhance the reproductive capacity and fermentation abilities of probiotics. Wu Qiuyu et al. 2011 ) Reports indicate that fumaric acid can promote the proliferation of beneficial bacteria in the animal gastrointestinal tract, enhance their microbial fermentation capacity, inhibit harmful bacteria, and maintain the gut's normal barrier function, thereby boosting the body's disease-resistance capabilities.

There are inconsistent reports regarding the effects of acidifiers on the gut microbiota of weaned piglets. Gedek 、 Kirehgessner And Franco The research results all indicate that after feed acidification, the number of harmful microorganisms in piglets' intestines decreases. The sustained-release composite acidifier is primarily composed of multiple organic acids, which inherently possess strong antibacterial properties. Once released in the intestinal tract, these organic acids help restore and maintain the balance of the piglets' gut microbiota. For instance, Bolduan The study found that, after using conventional slow-release products, the gut microbiota of piglets remained unaffected, while