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Q8 - Gram Staining, Monoderms, Diderms and Firmicute Evolution

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Cell Theory, Prokaryotes & Eukaryotes Phylogenetic Trees & Cladistics

Gram-staining has long been used to classify bacteria. Bacteria stain positively by the Gram method if they have a thick-enough layer of exposed peptidoglycan on the surface of their cells.

However, Gram-positive and Gram-negative bacteria are not monophyletic groups and can include species with rather different cell morphologies.

Some microbiologists have proposed to classify bacteria into monoderms, which have only the cytoplasmic membrane, and diderms, which additionally have an outer membrane. However, the evolution of the outer membrane is still a matter of research.

One recent study has investigated the Firmicutes phylum. The result of this study is summarized in Figure 1.

Firmicutes phylogeny showing five groups (I-V) with membrane diagrams above each. Groups I (Negativicutes) and IV (Halanaerobiales) are highlighted in magenta, indicating they are diderms (having an outer membrane, shown in purple). Groups II (Clostridiales/Thermoanaerobacterales), III (Bacillales/Lactobacillales), and V (Natranaerobiales) are monoderms (cytoplasmic membrane only, shown in blue with grey peptidoglycan). Figure 1. Phylogenetic relationship between various Firmicutes groups. Blue = cytoplasmic membrane, grey = peptidoglycan, purple = outer membrane. I = Negativicutes, II = Clostridiales and Thermoanaerobacterales, III = Bacillales and Lactobacillales, IV = Halanaerobiales, V = Natranaerobiales.

The study also showed that genes associated with the outer membrane are shared between Negativicutes and Halanaerobiales due to vertical transmission, and not due to horizontal gene transfer.

Q8.1.1. A typical diderm stains:
Q8.1.2. A typical monoderm stains:
Q8.1.3. A monoderm with no peptidoglycan on the surface stains:
Q8.2.1. Within Firmicutes, monoderms are:
Q8.2.2. Within Firmicutes, diderms are:
Q8.3.1. Based on these results, the most recent common ancestor of Firmicutes likely had the outer membrane.
Q8.3.2. Based on these results and the maximum parsimony principle, it is likely that Bacillales and Lactobacillales lost the outer membrane independently of each other.
Q8.3.3. These results disprove the hypothesis that the outer membrane evolved as a protection against penicillin-like antibiotics.
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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF