Bacterial Pathogens as Broken Bacteria

Summary

Gene loss and failed regulation are real parts of the history of bacterial disease.  They explain particular losses of independence and changes from less harmful to damaging relationships.  They do not establish that all pathogens began as harmless partners, and the evidence includes acquired capacities as well as deterioration.

• Leprosy supplies strong evidence of functional loss. A comparative reconstruction identified 1,537 lost functions among 2,977 inferred ancestral genes. Most remained as damaged DNA rather than being completely deleted.
• Dependence is not the same as disease. The reduced genome of the aphid partner Buchnera supports nutrient exchange that benefits its host.
• An established partnership can become harmful. Fruit-fly Wolbachia studies connect altered regulation with excessive bacterial growth and shorter host life. The original result concerned extra copies of existing DNA, not simple gene loss.
• Other bacterial histories are mixed. Yersinia comparisons implicate gains, losses, and changed ecology. Spirochete and Rickettsiales genomes support reduced independence but leave ancestral relationships partly reconstructed.

The creationist explanation is strongest when it identifies a particular damaged function or disrupted relationship.  Its universal historical conclusion requires more than those examples supply.  The broader framework is examined in Predation, Parasites, and Pathogens; this article tests the bacterial evidence.

What the creationist explanation actually proposes

A bacterium can become harmful by losing a useful function or by failing to control something it already does.  It can also cause disease when an existing activity occurs in a different host or place.  These are testable biological possibilities.  They become a much larger historical claim when used to explain the origin of every bacterial disease.

Creationist microbiologist Joseph Francis distinguished these possibilities in 2008.  His contribution to the Microbe Forum proposed genetic alteration and ecological displacement as separate explanations.  In the second, a microbe leaves the setting where its activity was beneficial and enters an organism with which it was not intended to interact.  Francis placed both changes after the Fall.  The biological mechanisms and that proposed date are distinct claims.¹

Warren Shipton's 2019 account is similarly broader than universal gene loss.  Alongside loss, its abstract invokes “the expression of front-loaded genes now facilitating pathological life cycles” and leaves other possibilities open.  He also considers changed ecological relationships and transferred genetic material, and acknowledges that important details remain unknown.  His discussion mainly concerns insect vectors and their parasites, but it states the general framework against which bacterial examples should be assessed.²

The strongest version of the bacterial argument therefore asks whether harmful relationships can arise through damage, uncontrolled growth, dependence, or changed context in previously functioning systems.  The answer is yes in particular cases.  The further claim that every pathogen descends from an originally harmless partnership requires evidence about each lineage's earlier relationships.  A list of missing genes cannot supply that history by itself.

The distinction matters because four different things can be called broken:

These distinctions let degeneration receive its proper evidential weight.  A loss can be real and biologically important without being a complete explanation of disease.

Leprosy: extensive damage, with a specific denominator

The bacterium associated with leprosy, Mycobacterium leprae, provides strong evidence of functional deterioration.  Its genome contains many damaged relatives of genes that still function in related bacteria.  This is more informative than merely finding that its genome is small: the surviving remnants help identify what was lost.³

In 2007, Laura Gómez-Valero and colleagues compared mycobacterial genomes to reconstruct the common ancestor of the leprosy and tuberculosis lineages.  Their rule was to count a gene as probably ancestral when it was present in at least two of the three main lineages examined.  They inferred an ancestral inventory of 2,977 genes and the loss of 1,537 of those functions in the leprosy lineage.  That is about 52% of the reconstructed inventory, not 52% of the living bacterium's DNA.³

The paper's categories make the result much clearer:

The first two rows add to 1,129 pseudogenes, damaged sequences retained in the genome.  The 408 absent genes had no significant detectable sequence similarity in the corresponding searches.  Even the absent category need not mean that every last nucleotide vanished: the authors discuss degraded remnants that may no longer be recognizable.  The defensible conclusion is extensive loss of ancestral functions, with much of the damaged DNA still present.³

There are also limits to the denominator.  The authors call the reconstructed ancestral inventory a minimum.  A gene lost independently from multiple descendants can disappear from the comparison, and genes acquired later in the leprosy lineage do not belong in that ancestral inventory.  Their numbers are a reconstruction made from the genomes and annotations available for that study.  They should not be combined with a different paper's gene count as though all studies used the same reference genome and counting rules.

What does this establish about disease?  It shows that a serious pathogen can have undergone massive deterioration of metabolic independence.  It does not show that all the lost functions were restraints whose removal caused disease.  Once a bacterium occupies a reliable host environment, functions for other environments can become dispensable and decay.  Gómez-Valero and colleagues explicitly discuss specialized host residence as a possible cause of the reduction.  The direction can therefore be dependence followed by loss, as well as loss that further increases dependence.³

Nor does the reconstruction identify a harmless original bacterium.  Its ancestral node connects the leprosy and tuberculosis lineages; reconstructing that node's gene content is different from reconstructing its effect on a host.  More metabolic self-sufficiency would not, by itself, demonstrate mutualism.

Finally, the study's use of recent does not mean a few thousand years.  Its dating framework places the inferred large inactivation episode within the last 20 million years.  The authors transferred a calibration from other bacteria and assumed comparable rates of change in a ribosomal gene.  They explicitly treated the result as an order-of-magnitude estimate, not a precise date.  Accepting the evidence of damaged genes while disputing that chronology requires a separate argument; the word recent supplies none.³

Reduced genomes can sustain a beneficial partnership

A useful comparison is Buchnera, the bacterial partner of aphids.  Shuji Shigenobu and colleagues reported the genome of strain APS in 2000.  Its chromosome contained 640,681 DNA base pairs, alongside two small plasmids, separate DNA molecules.  The chromosome figure is not the total for all three molecules.  The bacterium had a much smaller genetic inventory than its free-living relatives and depended on its host.⁴

Yet the association supplies essential nutrients to the aphid.  The study found genes for making amino acids that the animal needs but cannot make, alongside major gaps in other biosynthetic functions.  Host and symbiont supply complementary products.  The paper also identified extensive losses in regulatory systems, showing that missing regulation does not invariably turn a partner into a pathogen.⁴

This comparison does not prove that every ancestral bacterium was beneficial.  It establishes something narrower and essential: reduced self-sufficiency and benefit to a host can coexist.  The same broad signs often invoked for pathogenic degeneration, a small genome, missing metabolic functions, and dependence, occur in a cooperative association.  To establish harm, the evidence must include what happens to the host.

It also guards against treating independence as the only measure of biological success.  A dependent bacterium may do less on its own while contributing something valuable within a partnership.  Conversely, losing an activity may favor bacterial reproduction in one setting while damaging the host or restricting survival elsewhere.  The relevant question is which function changed, in which relationship, with what consequence.

Wolbachia: a partnership can become damaging

The fruit-fly Wolbachia studies provide more direct evidence of breakdown than a comparison of genome sizes.  These bacteria live inside host cells and are passed through mothers to offspring.  In the association studied by Ewa Chrostek and Luís Teixeira in 2015, ordinarily benign variants can protect fruit flies against viruses, whereas a harmful variant grows excessively and shortens fly life.⁵

The researchers connected a change in the number of copies of existing bacterial DNA with bacterial abundance and host survival.  That is evidence linking a genetic change to a damaging outcome within an established association.  It supports the central intuition behind a breakdown model: an existing partnership can become harmful without every component of the harmful interaction being newly invented.⁵

The original result nevertheless involved amplification, an increase in copies, rather than simple deletion.  Calling it gene loss would misdescribe the evidence.  Elves Duarte and colleagues' 2021 follow-up found a more complicated regulatory picture, with different genetic changes associated with excessive proliferation.  The effects also depended on environmental conditions and the host's developmental stage.⁶

A further result complicates the categories beneficial and harmful.  The life-shortening associations studied in 2021 also increased antiviral protection.  A host could therefore receive one benefit while suffering another cost.  That does not cancel the observed harm; it means the relationship cannot be classified from one trait alone.  Whether the host survives longer overall depends on the circumstances being compared.⁶

This is strong evidence for a particular transition toward harmfulness.  Its limits are equally clear.  It concerns an already established relationship between bacteria and flies.  It does not recover the first host of the lineage, identify the original purpose of every bacterial function, or date the beginning of bacterial disease.  Those larger claims do not follow simply because a breakdown is observed today.

Yersinia: losses matter within a history of gains and losses

The plague bacterium is another important case, but it answers a different question.  Yi-Cheng Sun and colleagues' 2014 study examined the transition toward flea-borne transmission in Yersinia pestis.  Its close comparator, Yersinia pseudotuberculosis, already causes intestinal disease.  This comparison therefore concerns a change between disease ecologies, not a demonstrated transition from complete harmlessness to disease.⁷

The study found that acquired capacities and losses of existing functions both contributed to the change.  It supports a real role for loss in the emergence of a more specialized pathogen.  Losing a function can remove a constraint and alter what the bacterium does in a particular environment.  But the result cannot honestly be summarized as loss instead of gain, and the focal transmission changes are not a complete account of every feature of plague.⁷

A broader comparison by Sandra Reuter and colleagues, also published in 2014, examined 241 Yersinia strains across the genus.  Their analysis included environmental relatives rather than comparing only clinical isolates.  They inferred independent emergence of pathogenic lineages, accompanied by acquisition of disease-associated capacities and increasing metabolic restriction.  The study is valuable precisely because it supplies the nonpathogenic comparisons missing from a pathogen-only sample.⁸

The two papers operate at different scales.  Sun and colleagues examined a particular transition within a close relationship.  Reuter and colleagues reconstructed a wider family history.  The broader result makes origins from environmental lineages plausible in specific cases; it does not turn the immediate comparator in the plague study into a harmless organism.

The combined finding fits part of the creationist alteration model: narrower metabolism and loss of functions can contribute to disease-associated specialization.  It also fits the evolutionary interpretation advanced by the researchers, in which selection, loss, acquisition, and ecological change alter established bacterial lineages.  Identifying a loss does not decide between those accounts, because both allow losses.  The disagreement concerns the larger history, including the relationships and capacities of earlier organisms.

What the spirochetes and Rickettsiales add

Syphilis and Lyme disease: dependence is real, subtraction is not reconstruction

The original genome papers support metabolic dependence in the bacteria associated with syphilis and Lyme disease.  They do not establish a universal number of genes required for independent bacterial life, or a direct ledger of thousands of genes lost from an identified harmless ancestor.⁹¹⁰

Claire Fraser and colleagues' 1998 syphilis paper reported 1,041 predicted coding sequences in the Treponema pallidum Nichols strain.  Its DNA comprised 1,138,006 base pairs.  The authors found reduced metabolic activities alongside functioning systems for copying and using genetic information.  Their discussion of its proposed history includes loss of unnecessary genes and acquisition of functions useful in a host environment.  The count belongs to that strain and the study's original annotation, not every spirochete or every subsequent annotation.¹⁰

The 1997 Borrelia burgdorferi B31 study shows why even a rough comparison needs care.  It reported 853 predicted genes on the main chromosome and 430 putative coding regions on the eleven plasmids described.  Additional plasmids were recognized but not all confidently assembled.  The plasmid regions also included apparent decaying sequences.  Thus neither the chromosome alone nor a simple sum is a timeless count of all functional genes in the organism.⁹

Those details matter for the claim, repeated in popular discussion, that these disease bacteria have about a thousand genes while free-living relatives have about five thousand.  As a rough contrast in self-sufficiency, it points toward a real biological difference.  As a claim that exactly four thousand ancestral genes disappeared, it is inadequate.  That calculation needs a named ancestor, comparable annotations, evidence that the extra genes were ancestral, and a distinction between missing functions and functions supplied by the host.  The leprosy reconstruction attempts this kind of comparison; rough subtraction between different living bacteria does not.

A bacterium's ability to obtain nutrients from host tissue also does not tell whether its earlier association helped the host.  It establishes a dependence that requires explanation.  Demonstrating original mutualism would require evidence of what the proposed ancestor supplied in return.

Rickettsiales: possible repurposing, with an uncertain starting relationship

Max Schön and colleagues' 2022 study supplies a stronger comparative approach to the origins of host association.  They reconstructed eleven bacterial genomes from environmental DNA and identified three previously undescribed groups related to Rickettsiales, a group containing intracellular partners and pathogens.  Some of the newly recovered relatives possessed a richer inferred metabolism than the familiar host-dependent members.¹¹

Their ancestral reconstruction favored a free-living or optionally host-associated starting point, followed by increasing dependence.  The authors also proposed that an existing system for interaction with other cells was repurposed during the transition toward host association.  This gives reuse and reduction a concrete comparative basis, rather than invoking an unspecified earlier function whenever a harmful system is encountered.¹¹

Several qualifications remain.  These were genomes assembled from environmental samples, not observations of a resurrected ancestor.  The researchers explicitly left optional host association open.  Their proposed earlier uses included competition against other bacteria and defense against protist predation, so earlier function is not synonymous with universal harmlessness.  The inferred history also included acquired capacities alongside losses.¹¹

The appropriate conclusion is that the study supports an earlier, more metabolically independent condition and a possible history of repurposing.  It does not establish that all ancestral relationships were cooperative, or that every later disease system resulted solely from deterioration.

Objections and responses

If a pathogen lost functions, why not simply call it broken?

That is reasonable when broken identifies the particular function that no longer works.  In the leprosy case, extensive functional degradation is well supported.  The ambiguity arises when the same word is made to explain host damage, establish original mutualism, and date the transition.  Each of those conclusions needs additional evidence.  A bacterium can be impaired in independent metabolism and effective at reproducing within its host.

Does bacterial evolution therefore explain away the creationist claim?

It explains and tests parts of the same biological history that the creationist claim addresses.  The observations include loss, regulatory change, acquired capacities, and altered ecological relationships.  Francis and Shipton already allow more than a single mechanism.  Showing that their proposed mechanisms occur supports their biological plausibility; it does not establish their complete historical or theological explanation.¹²

Conversely, a mechanism occurring in an evolutionary reconstruction does not make the observed deterioration unreal.  The damaged genes remain evidence that particular capacities were lost.  The relevant disagreement is over what further conclusions that evidence can support.

Could the acquired capacities also come from an originally good system?

They could, in principle.  Acquiring DNA from another organism does not establish how that DNA first originated or what all its earlier uses were.  But a proposed benign origin becomes an explanation only when it identifies a plausible earlier role and shows how the later relationship arose.  The Rickettsiales work provides a specific repurposing hypothesis with acknowledged uncertainties; universal original cooperation remains a broader claim.¹¹

Does a beneficial relative prove that the pathogen used to be beneficial?

It provides a useful comparison, not a verdict about ancestry.  The host relationship could have changed on either branch, and living relatives are not automatically one another's ancestors.  The Wolbachia work is unusually informative because it links changed bacterial behavior to changed host outcomes within an existing association.  Buchnera demonstrates that dependence can be beneficial; it does not establish the ancestral state of leprosy or plague.⁴⁵

If the harm follows a change in location, must the bacterium be damaged at all?

No.  That is why Francis separated displacement from genetic alteration.  A biological activity can be useful in one relationship and damaging in another.  For a particular disease, however, the proposed earlier relationship still needs evidence.  Labeling an organism displaced without identifying a plausible prior setting only restates the hypothesis.¹

Do rapid changes establish an origin after the Fall?

No calendar date follows from the speed of an observed change alone.  Rapid change shows that a transition need not always be slow; it does not establish when a historical transition occurred.  The leprosy paper's own chronological model is far older than a young-earth chronology.  A post-Fall account must supply and defend its own historical constraints instead of silently adopting the study's mechanism and replacing its dates.³

Issues and unanswered questions

Three questions remain especially important.  First, which losses initiated harmfulness, and which accumulated after dependence was established?  Comparative genomes often reveal missing functions more securely than the order of ecological events.  Second, what did the proposed ancestors actually do to their hosts or neighbors?  A reconstructed metabolic inventory does not directly measure a past relationship.  Third, can a creationist account connect these separate bacterial histories to a common recent chronology with evidence independent of the theological premise?

The examples here do not cover all bacterial diseases.  They were chosen because the relevant claims have identifiable comparative or experimental evidence.  They demonstrate that breakdown, reduction, and repurposing deserve a place in an explanation of disease.  They also show why those mechanisms cannot be converted into a universal history by changing the meaning of broken from one paragraph to the next.

The defensible verdict is a qualified one: some bacterial pathogens are damaged or poorly regulated descendants of more self-sufficient or less harmful forms, but bacterial disease is not explained by gene loss alone.  Beneficial dependence, retained harmful capacities, acquisitions, and changed relationships belong in the same account.  The evidence supports particular losses and particular breakdowns; universal ancestral harmlessness and a post-Fall date remain additional claims to establish.

Sources