Gram-negative bacteria do not form true endospores. Endospore formation is restricted to the phylum Firmicutes, a group historically defined as Gram-positive bacteria with thick cell walls. However, the full picture has a fascinating wrinkle: a small number of Firmicutes species actually stain Gram-negative and possess a double-membrane cell envelope, yet they still produce genuine endospores. These organisms blur the boundary between the two categories, but they don’t change the core rule.
Why Endospores Belong to Firmicutes
Endospore formation requires a highly specialized set of genes, including a master regulator called Spo0A and a cascade of signaling proteins that coordinate the process. These sporulation genes are universally conserved among spore-forming Firmicutes but are absent from the genomes of true Gram-negative bacteria like E. coli, Salmonella, or Pseudomonas. Without this genetic machinery, a bacterium simply cannot build an endospore.
A 2021 study in Applied and Environmental Microbiology directly tested the two most prominent claims that Gram-negative Proteobacteria could sporulate. Researchers used advanced microscopy, biochemical analysis, and genome screening to examine Rhodobacter johrii and Serratia marcescens, both previously reported as endospore formers. The phase-bright structures earlier identified as spores turned out to be storage granules and cellular debris. Neither species could form true endospores, and their genomes lacked the necessary sporulation genes. The study’s conclusion was unambiguous: endospore formation remains unique to Firmicutes.
The Gram-Negative Firmicutes Exception
Biology rarely draws perfectly clean lines. Within the Firmicutes, a family called Veillonellaceae includes species that look and behave like Gram-negative bacteria in important ways: they stain Gram-negative, they have both an inner and outer membrane, a thin cell wall, and their outer membranes contain lipopolysaccharide (LPS), the same molecule found on classic Gram-negative bacteria. Yet these organisms are genetically Firmicutes and carry functional sporulation genes.
The best-studied example is Acetonema longum, an anaerobic bacterium whose sporulation process has been captured in detailed 3D electron microscopy. During sporulation, the mother cell engulfs the developing spore in a process resembling how immune cells swallow pathogens. When the spore later germinates, the inner membrane of the original mother cell inverts and becomes the outer membrane of the new vegetative cell. This means the double-membrane “Gram-negative” envelope is actually a product of the sporulation cycle itself. Some researchers view A. longum as a “missing link” that hints at how the bacterial outer membrane may have evolved in the first place.
So while a handful of bacteria with Gram-negative cell envelopes can form endospores, they are Firmicutes by lineage. No member of the classically Gram-negative phyla (Proteobacteria, Bacteroidetes, and others) has ever been confirmed to sporulate.
What Makes Endospores So Resistant
The reason this question matters practically is that endospores are extraordinarily tough. A Bacillus subtilis spore can survive boiling water at 100°C, requiring 20 to 30 minutes just to reduce the viable population by 90%. In dry heat, spores survive roughly 1,000 times longer than in moist heat. Killing them requires temperatures 30 to 40°C higher than what’s needed to kill actively growing cells of the same species. This resistance extends to UV radiation, extreme pH, and high pressure.
Spore-forming Firmicutes are remarkably diverse. They include common soil organisms like Bacillus, salt-loving species like Sporohalobacter, sulfate reducers like Desulfotomaculum, and even photosynthetic bacteria like Heliobacterium. What they all share is that sporulation kit of genes, triggered when nutrients run low.
Survival Tricks That Aren’t Endospores
Some Gram-negative bacteria have evolved their own impressive survival strategies that can look superficially similar to sporulation but work through entirely different mechanisms.
Coxiella burnetii, the Gram-negative bacterium that causes Q fever, produces a “small cell variant” (SCV) as part of a two-phase life cycle. This compact, dense form resists desiccation, UV light, elevated temperatures, osmotic shock, and chemical disinfectants well enough that fewer than 10 organisms can cause infection through aerosol exposure. Early microscopy sometimes revealed a dense polar body inside the cell that resembled an endospore, but genome analysis confirmed the organism lacks sporulation gene homologs. The SCV’s resistance comes from structural compaction and changes in cell wall composition, not from the engulfment and coat-building process that defines true endospores.
Myxobacteria, another Gram-negative group, respond to starvation by aggregating into fruiting bodies containing tough, dormant cells called myxospores. Actinobacteria produce exospores, which bud from the cell surface rather than forming internally. These structures preserve genetic material under harsh conditions, but none match the extreme heat and radiation resistance of a true Firmicutes endospore.
How to Tell the Difference
True endospores have a defining feature: they form inside the parent cell through an engulfment process that wraps the developing spore in two membranes, then adds layers of protective coat proteins. Under a phase-contrast microscope, they appear as bright, refractile bodies within the cell. They resist standard staining and require special techniques like the Schaeffer-Fulton stain (malachite green with heat) to visualize.
If you encounter a claim that a Gram-negative organism forms endospores, the key tests are genomic (does it carry Spo0A and the sporulation sigma factors?) and structural (does microscopy confirm a true engulfment-derived spore with a multilayered coat?). Storage granules, inclusion bodies, and dense resting forms can all mimic the appearance of spores under basic microscopy, which is exactly what tripped up the earlier reports on Proteobacteria.
The bottom line: if a bacterium genuinely makes endospores, it belongs to the Firmicutes, even if its cell envelope happens to look Gram-negative.