Have you ever eaten something that tasted completely fine, but later you started wondering if it was actually safe? Food contamination is one of those things people do not think about until it happens. Even with modern sterilization and food processing, outbreaks still occur. According to the CDC’s food safety overview, about 48 million people in the United States get a foodborne illness each year, 128,000 are hospitalized, and 3,000 die. Contamination can occur if food is not cooked properly, handled safely, or refrigerated correctly. Some microbes are especially hard to control, they can wait until conditions improve.
One especially important example is Bacillus subtilis. Instead of trying to survive stress as a normal cell, it can form a spore, an extremely resistant form that survives heat, dryness, and acidity. That means even when food looks safe, spores can still be present. Large food systems face huge risks from this persistence. A recent example is the CDC investigation of the 2024–2025 Listeria outbreak linked to meats sliced at delis. In that outbreak, CDC reported 61 cases, 60 hospitalizations, and 10 deaths across 19 states. Contamination can spread in ways that are hard to detect, highlighting why food processing environments are monitored closely.
That is part of why I found this paper so interesting. A recent study, “Sporulation kinetics of Bacillus subtilis under pH shift conditions: impact on spore properties and gene expression”, asked a very practical question. What happens when the environment suddenly becomes more acidic? Can Bacillus subtilis still form spores and survive, or is there a point where the stress becomes too much? The answer turns out to be very sharp. A small shift in pH can completely change the bacterium’s fate.
Getting to Know the Microbe
Bacillus subtilis is a common soil bacterium and a widely used model organism because it is well studied and easy to work with in the lab. It also matters in real environments, including food systems, because its spores can survive harsh conditions. While B. subtilis itself rarely causes foodborne illness, studying it helps scientists understand how spores form, survive, and germinate, which is important for managing risks from harmful spore-formers like Bacillus cereus and Clostridium species (NCBI).
This image shows Bacillus cells under phase-contrast microscopy. The bright, shiny spots are spores, which stand out from the darker vegetative cells because of their dense structure. This is exactly how researchers identify when bacteria have entered the spore state, which is important because these spores are the forms that survive harsh conditions like heat and acid.
This diagram shows the structure of a bacterial endospore. The core contains the DNA, while layers like the cortex and spore coat protect it from damage. These layers are what make spores so resistant, allowing Bacillus subtilis to survive food processing conditions that would normally kill other cells.
The spores themselves are incredibly resistant. In this study, the researchers counted spores as cells that survived a heat treatment of 80°C for 10 minutes. That is already enough to show why spores are such a headache. A process that kills ordinary cells may still leave spores behind, and those spores can later germinate and grow again. This also helps explain why contamination is not always easy to confirm. Food can look fine and still contain dangerous microbes or dormant spores.
What Happens When the Environment Suddenly Changes
A lot of microbiology experiments use stable lab conditions, but real food systems are not stable. pH can change during fermentation, cleaning, storage, and processing. The authors of this paper wanted to model that kind of shift.
They first grew Bacillus subtilis in bioreactors at pH 7.0. After 16 hours, which corresponded to the end of the exponential growth phase, they suddenly shifted the cultures to either pH 5.5 or pH 4.0. That design is clever because it separates the effects on sporulation from the earlier growth phase. The strain’s minimum pH for growth was estimated at 4.58, so pH 5.5 was still within the survivable range, while pH 4.0 was below it. This is where the study becomes much more than a simple growth experiment. The researchers were not only measuring how many spores formed but were also tracking gene expression, heat resistance, and germination. They wanted to know what kind of spores were produced and what was happening inside the cells while that decision was being made. Does acid stress delay sporulation, or does it stop it completely?
Figure 1. Growth and spore formation of B. subtilis under different pH conditions. (A) At pH 7.0, growth and sporulation were measured and fitted to the data (dashed lines).(B) When the pH was shifted from 7.0 to 5.5 after 16 h, growth and spore formation slowed at first (solid lines). (C) When the pH was shifted from 7.0 to 4.0, sporulation was mostly blocked. Total cells are shown in black, and heat-resistant spores are shown in red (A), blue (B), and green (C). Vertical dashed lines mark when the pH shift happened. Figure 1 credit: adapted from Hafdane et al., 2025, Applied and Environmental Microbiology.
The data tell a very clear story. When the pH shifted from 7.0 to 5.5, sporulation still occurred, but it slowed down considerably (Figure 1B). After the shift, spore counts remained around 8.5 × 10³ CFU/mL for about 22 hours before increasing again, eventually reaching 4.2 × 10⁸ CFU/mL after 125 hours. This shows that the cells were stressed by the pH change, but they were able to pause, adapt, and complete sporulation. At pH 4.0, the result was completely different. After the pH shift, spore counts stayed at about 2 log₁₀ CFU/mL and did not increase. The paper describes sporulation as completely inhibited under these conditions. That matters because it shows a real threshold. There is a point where the process shuts down entirely.
The Genes And The Spores
The really interesting part is that the cells do not just “look stressed.” Their genetic program changes in a way that explains the outcome.
At pH 5.5, the paper reports prolonged expression of sporulation genes. That fits the delayed timeline seen in Figure 1. The cells are still committed to sporulation, but the process stretches out. At pH 4.0, key sporulation genes become transcriptionally silent. Once that happens, the developmental pathway cannot continue. The cells are not just slowed down; they are unable to activate the machinery needed to make spores, which is why I like this paper. It does not just say that acid affects sporulation, it links the visible outcome to the internal molecular response, making the result much more convincing.
The paper also looked beyond the number of spores and asked whether the spores made under different conditions behaved differently.
Interestingly, spores produced at pH 7.0 and under the pH 7.0 to 5.5 shift had similar heat resistance. So the mildly acidic environment did not make them obviously weaker in terms of surviving heat. But germination was different. Spores produced under the pH 5.5 shift germinated more efficiently in L-alanine suspension (L‑alanine is an amino acid that is dissolved in water to make an “L‑alanine suspension,” which scientists use in experiments to trigger bacterial spore germination) than spores produced at pH 7.0. That means the environment during sporulation does not just affect whether spores form. It also affects what those spores are like later. That part feels especially relevant for food microbiology. If stressful environments create spores with altered germination behavior, then processing conditions might shape not only immediate survival but what happens after the food leaves the facility.
Why This Matters Outside the Lab
Acidity is a key tool in food preservation: fermentation, pickling, and other low-pH processes rely on it. But “acidic” is not one-size-fits-all. Moderate acidity can allow spores to form, while stronger acidity can block them. Small differences in conditions can create very different outcomes, making contamination trickier to manage. The study also reminds us that microbes exist in different states: active cells, dormant spores, or in transition. Each state affects how hard they are to kill and how likely they are to come back.
What I Would Want to See Next
The abrupt pH changes in this study are a good start, but real food often acidifies gradually. Testing slower changes or combining stressors like pH plus temperature would be interesting. Extending the research to pathogenic spore-formers would make the implications for food safety even stronger.
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